Manufacturing method of semiconductor device

ABSTRACT

Disclosed is a semiconductor device using an oxide semiconductor, with stable electric characteristics and high reliability. In a process for manufacturing a bottom-gate transistor including an oxide semiconductor film, dehydration or dehydrogenation is performed by heat treatment and oxygen doping treatment is performed. The transistor including a gate insulating film subjected to the oxygen doping treatment and the oxide semiconductor film subjected to the dehydration or dehydrogenation by the heat treatment is a transistor having high reliability in which the amount of change in threshold voltage of the transistor by the bias-temperature stress (BT) test can be reduced.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/591,043, filed Jan. 7, 2015, now allowed, which is a continuation ofU.S. application Ser. No. 13/091,190, filed Apr. 21, 2011, now U.S. Pat.No. 8,945,982, which claims the benefit of a foreign priorityapplication filed in Japan as Serial No. 2010-100316 on Apr. 23, 2010,all of which are incorporated by reference.

TECHNICAL FIELD

The present invention relates to a semiconductor device and amanufacturing method thereof.

In this specification, a semiconductor device generally means any devicewhich can function by utilizing semiconductor characteristics, and anelectrooptic device, a semiconductor circuit, and electronic equipmentare all semiconductor devices.

BACKGROUND ART

A technique by which transistors are formed using semiconductor thinfilms formed over a substrate having an insulating surface has beenattracting attention. The transistor is applied to a wide range ofelectronic devices such as an integrated circuit (IC) or an imagedisplay device (display device). A silicon-based semiconductor materialis widely known as a material for a semiconductor thin film applicableto the transistor; in addition, an oxide semiconductor has beenattracting attention as another material.

For example, a transistor whose active layer includes an amorphous oxidecontaining indium (In), gallium (Ga), and zinc (Zn) and having anelectron carrier concentration of less than 10¹⁸/cm⁻³ is disclosed (seePatent Document 1).

REFERENCE

Patent Document 1: Japanese Published Patent Application No. 2006-165528

DISCLOSURE OF INVENTION

However, when hydrogen or moisture, which forms an electron donor, ismixed into the oxide semiconductor in a process for manufacturing adevice, the electrical conductivity of the oxide semiconductor maychange. Such a phenomenon causes variation in the electriccharacteristics of a transistor using the oxide semiconductor.

In view of such a problem, one object of one embodiment of the presentinvention is to provide a semiconductor device using an oxidesemiconductor, with stable electric characteristics and highreliability.

In a process for manufacturing a transistor including an oxidesemiconductor film, dehydration or dehydrogenation is performed by heattreatment and oxygen doping treatment is performed. According to oneembodiment of the present invention, oxygen doping treatment isperformed at least on a gate insulating film.

One embodiment of the present invention is a method for manufacturing asemiconductor device, in which a gate electrode layer is formed, a gateinsulating film is formed over the gate electrode layer, oxygen dopingtreatment is performed on the gate insulating film to supply an oxygenatom to the gate insulating film, an oxide semiconductor film is formedover the gate insulating film so as to overlap with the gate electrodelayer, heat treatment is performed on the oxide semiconductor film toremove a hydrogen atom in the oxide semiconductor film, a sourceelectrode layer and a drain electrode layer which are electricallyconnected to the oxide semiconductor film are formed, and an insulatingfilm is formed over the oxide semiconductor film, the source electrodelayer, and the drain electrode layer so as to be in contact with theoxide semiconductor film.

Note that the above-described “oxygen doping” means that oxygen (whichincludes at least one of an oxygen radical, an oxygen atom, and anoxygen ion) is added to a bulk. Note that the term “bulk” is used inorder to clarify that oxygen is added not only to a top surface of athin film but also to the inside of the thin film. In addition, “oxygendoping” includes “oxygen plasma doping” in which oxygen in the form ofplasma is added to a bulk.

By the oxygen doping treatment in the manufacturing process of thetransistor including the oxide semiconductor film, an oxygen-excessiveregion where the amount of oxygen is greater than the stoichiometricproportion can be provided in at least one of the gate insulating film(bulk thereof), the oxide semiconductor film (bulk thereof), theinsulating film (bulk thereof), an interface between the gate insulatingfilm and the oxide semiconductor film, and an interface between theoxide semiconductor film and the insulating film. The amount of oxygenis preferably greater than the stoichiometric proportion and less thanfour times of the stoichiometric proportion, far preferably greater thanthe stoichiometric proportion and less than double of the stoichiometricproportion. Such an oxide including excessive oxygen whose amount isgreater than the stoichiometric proportion refers to, for example, anoxide which satisfies 2g>3a+3b+2c+4d+3e+2f (g is greater than the sum of1.5a+1.5b+c+2d+1.5e+1), where the oxide is represented asIn_(a)Ga_(b)Zn_(c)Si_(d)Al_(e)Mg_(f)O_(g) (a, b, c, d, e, f, g≧0: a, b,c, d, e, f, g is greater than zero). Note that oxygen which is added bythe oxygen doping treatment may exist between lattices of the oxidesemiconductor.

The above-described oxygen-excessive region may be provided in two ormore of the gate insulating film, the oxide semiconductor film, and theinsulating film. For example, oxygen-excessive regions can be providedin the interface between the gate insulating film and the oxidesemiconductor film, the oxide semiconductor film (bulk thereof), and theinterface between the oxide semiconductor film and the insulating filmby oxygen doping treatment in the manufacturing process.

Note that while it is acceptable that the amount of oxygen is equal tothe stoichiometric proportion in a defect- (oxygen deficiency-)freeoxide semiconductor, in order to secure reliability, for example, tosuppress variation in the threshold voltage of a transistor, it ispreferable that an oxide semiconductor include oxygen whose amount begreater than the stoichiometric proportion. Similarly, while the basefilm is not necessarily an insulating film containing excessive oxygenin the case of a defect- (oxygen deficiency-)free oxide semiconductor,in order to secure reliability, for example, to suppress variation inthe threshold voltage of a transistor, it is preferable that the basefilm be an insulating film containing excessive oxygen, considering thepossibility of occurrence of oxygen deficiency in the oxidesemiconductor layer.

With the dehydration or dehydrogenation by the heat treatment subjectedto the oxide semiconductor film, a hydrogen atom or an impuritycontaining a hydrogen atom such as water in the oxide semiconductor filmis removed, so that the oxide semiconductor film is highly purified. Theamount of oxygen added by the oxygen doping treatment is set to greaterthan the amount of hydrogen in the highly-purified oxide semiconductorfilm which has been subjected to the dehydration or dehydrogenation.Excessive oxygen in at least one of the stacked gate insulating film,oxide semiconductor film, and insulating film diffuses and reacts withhydrogen that causes instability, thereby fixing hydrogen (makinghydrogen an immovable ion). That is, instability in the reliability canbe reduced (or sufficiently decreased). In addition, with excessiveoxygen, variation in the threshold voltage Vth due to oxygen deficiencycan be reduced and the amount of shift ΔVth of the threshold voltage canbe reduced.

Here, a state in which oxygen is added to the bulk by theabove-described “oxygen plasma doping” treatment is described. Note thatwhen oxygen doping treatment is performed on an oxide semiconductor filmcontaining oxygen as one component, it is generally difficult to checkan increase or a decrease of the oxygen concentration. Therefore, here,an effect of the oxygen doping treatment was confirmed with a siliconwafer.

Oxygen doping treatment was performed with the use of an inductivelycoupled plasma (ICP) method. Conditions thereof were as follows: the ICPpower is 800 W; the RF bias power 300 W or 0 W; the pressure 1.5 Pa; theoxygen gas flow rate 75 sccm; and the substrate temperature 70° C. FIG.15 shows an oxygen concentration profile in the depth direction of thesilicon wafer according to secondary ion mass spectrometry (SIMS)measurement. In FIG. 15, the vertical axis indicates an oxygenconcentration; the horizontal axis indicates a depth from a top surfaceof the silicon wafer.

It can be confirmed from FIG. 15 that oxygen is added in either of caseswhere the RF bias power is 0 W or the RF bias power is 300 W. Inaddition, in the case where the RF bias power is 300 W, oxygen is addedmore deeply than the case of the RF bias power of 0 W.

Next, FIGS. 16A and 16B show results of observation of a cross sectionof the silicon wafer before and after the oxygen doping treatmentaccording to scanning transmission electron microscopy (STEM). FIG. 16Ais a STEM image of the silicon wafer before the oxygen doping treatment.FIG. 16B is a STEM image of the silicon wafer after the oxygen dopingtreatment at the RF bias power of 300 W. Referring to FIG. 16B, it canbe confirmed that an oxygen-highly-doped region is formed in the siliconwafer by the oxygen doping.

As described above, it is shown that oxygen is added to the siliconwafer by oxygen doping on the silicon wafer. This result leads to anunderstanding that oxygen can also be added to an oxide semiconductorfilm by oxygen doping to the oxide semiconductor film.

The effect of the structure which is an embodiment of the invention canbe easily understood by considering as below. The description below isjust one exemplary consideration.

When a positive voltage is applied to the gate electrode, an electricfield is generated from a gate electrode side of the oxide semiconductorfilm to a back channel side (the opposite side to the gate insulatingfilm), and accordingly, hydrogen ions having positive charge which existin the oxide semiconductor film are transferred to the back channelside, and accumulated in an oxide semiconductor film side of aninterface between the oxide semiconductor film and the insulating film.The positive charge is transferred from the accumulated hydrogen ion toa charge trapping center (such as a hydrogen atom, water, orcontamination) in the insulating film, whereby negative charge isaccumulated in the back channel side of the oxide semiconductor film. Inother words, a parasitic channel is generated on the back channel sideof the transistor, and the threshold voltage is shifted to the negativeside, so that the transistor tends to be normally-on.

In this manner, the charge trapping center such as hydrogen or water inthe insulating film traps the positive charge and is transferred intothe insulating film, which varies electric characteristics of thetransistor. Therefore, in order to suppress variation of the electricalcharacteristics of the transistor, it is important that there is nocharge trapping center or the number of charge trapping centers is smallin the insulating film. Therefore, a sputtering method by which lesshydrogen is contained in film deposition is preferably used forformation of the insulating film. In an insulating film deposited by thesputtering method, there is no charge trapping center or the number ofwhich is small, and the transfer of positive charge less occurs ascompared to that in the case of using a CVD method or the like.Accordingly, the shift of the threshold voltage of the transistor can besuppressed and the transistor can be normally off.

On the other hand, when a negative voltage is applied to the gateelectrode, an electric field is generated from the back channel side tothe gate electrode side, and accordingly, hydrogen ions which exist inthe oxide semiconductor film are transferred to the gate insulating filmside and are accumulated in the oxide semiconductor film side of theinterface between the oxide semiconductor film and the gate insulatingfilm. As a result, the threshold voltage of the transistor is shifted tothe negative side.

In a state being applied with a voltage of 0, the positive charge isreleased from the charge trapping center, so that the threshold voltageof the transistor is shifted to the positive side, thereby returning tothe initial state, or the threshold voltage is shifted to the positiveside beyond the initial state in some cases. These phenomena indicatethe existence of easy-to-transfer ions in the oxide semiconductor film.It can be considered that an ion that is transferred most easily is ahydrogen ion that is the smallest atom.

Note that in a bottom-gate transistor, when an oxide semiconductor filmis formed over a gate insulating film and then heat treatment isperformed thereon, not only water or hydrogen contained in the oxidesemiconductor film but also water or hydrogen contained in the gateinsulating film can be removed. Thus, in the gate insulating film, thenumber of charge trapping centers for trapping positive charge which istransferred through the oxide semiconductor film is small. In thismanner, the heat treatment for dehydration or dehydrogenation isperformed not only on the oxide semiconductor film but also on the gateinsulating film below the oxide semiconductor film. Therefore, in thebottom-gate transistor, the gate insulating film may be formed by a CVDmethod such as a plasma CVD method.

In addition, the oxide semiconductor film absorbs light, whereby a bondof a metal element (M) and a hydrogen atom (H) (the bond also referredto as an M-H bond) in the oxide semiconductor film is cut by opticalenergy. Note that the optical energy having a wavelength of about 400 nmis equal to or substantially equal to the bond energy of a metal elementand a hydrogen atom. When a negative gate bias is applied to atransistor in which the bond of a metal element and a hydrogen atom inthe oxide semiconductor film is cut, a hydrogen ion detached from themetal element is attracted to a gate electrode side, so thatdistribution of electrical charge is changed, the threshold voltage ofthe transistor is shifted to the negative side, and the transistor tendsto be normally on.

Note that the hydrogen ions which have been transferred to the interfacewith the gate insulating film by light irradiation and application ofthe negative gate bias to the transistor are returned to the initialstate by stopping application of the voltage. This can be regarded as atypical example of the ion transfer in the oxide semiconductor film.

In order to prevent such a change of the electrical characteristics byvoltage application (BT deterioration) or a change of the electricalcharacteristics by light irradiation (light deterioration), it isimportant to remove a hydrogen atom or an impurity containing a hydrogenatom such as water thoroughly from the oxide semiconductor film tohighly purify the oxide semiconductor film. The charge density as smallas 10¹⁵ cm⁻³, or the charge per unit area as small as 10¹⁰ cm⁻² does notaffect the transistor characteristics or very slightly affects them.Therefore, it is preferable that the charge density be less than orequal to 10¹⁵ cm⁻³. Assuming that 10% of hydrogen contained in the oxidesemiconductor film is transferred within the oxide semiconductor film,it is preferable that the hydrogen concentration be less than or equalto 10¹⁶ cm⁻³. Further, in order to prevent entrance of hydrogen from theoutside after a device is completed, it is preferable that a siliconnitride film formed by a sputtering method be used as a passivation filmto cover the transistor.

Hydrogen or water can also be removed from the oxide semiconductor filmby doping with excessive oxygen as compared to hydrogen contained in theoxide semiconductor film (such that (number of hydrogen atoms)<<(numberof oxygen radicals) or (number of oxygen ions)). Specifically, oxygen ismade to be plasma by a radio-frequency wave (RF), the bias of thesubstrate is increased, and an oxygen radical and/or an oxygen ionare/is doped or added into the oxide semiconductor film over thesubstrate such that the amount of oxygen is greater than that ofhydrogen in the oxide semiconductor film. The electronegativity ofoxygen is 3.0 which is larger than about 2.0, the electronegativity of ametal (Zn, Ga, In) in the oxide semiconductor film, and thus, excessiveoxygen which is excessive as compared to hydrogen is made to becontained to abstract hydrogen from the M-H group of a hydrogen atom, sothat an OH group is formed. This OH group may form an M-O—H group with abond to M.

The oxygen doping is preferably performed such that the amount of oxygenin the oxide semiconductor film is greater than the stoichiometricproportion. For example, in the case where an In—Ga—Zn—O-based oxidesemiconductor film is used as the oxide semiconductor film, an idealsingle-crystal ratio is 1:1:1:4 (InGaZnO₄), and therefore, it is farpreferable that the amount of oxygen be made to greater than thestoichiometric proportion and less than double of the stoichiometricproportion by oxygen doping or the like. Accordingly, the amount ofoxygen is greater than that of hydrogen in the oxide semiconductor film.

Optical energy or BT stress abstracts hydrogen from the M-H group, whichcauses deterioration; however, in the case where oxygen is added by theabove-described doping, added oxygen is bonded with a hydrogen ion, sothat an OH group is formed. The OH group does not discharge a hydrogenion even by light irradiation or application of BT stress on thetransistor because of its high bond energy, and is not easilytransferred into the oxide semiconductor film because of its greatermass than the mass of a hydrogen ion. Accordingly, an OH group formed byoxygen doping does not cause deterioration of the transistor or cansuppress the deterioration.

In addition, it has been confirmed that as the thickness of the oxidesemiconductor film is increased, the variation in the threshold voltageof a transistor tends to increase. It can be guessed that this isbecause an oxygen defect in the oxide semiconductor film is one cause ofthe change of the threshold voltage and increases as the thickness ofthe oxide semiconductor film is increased. A step of doping an oxidesemiconductor film with oxygen in a transistor according to oneembodiment of the present invention is effective not only for removal ofhydrogen or water from the oxide semiconductor film but also forcompensation of an oxygen defect in the film. Accordingly, the variationin the threshold voltage can also be controlled in the transistoraccording to one embodiment of the present invention.

Metal oxide films each formed of a component/components similar to theoxide semiconductor film may be provided with the oxide semiconductorfilm provided therebetween, which is also effective for prevention ofchange of the electrical characteristics. As the metal oxide film formedof a component/components similar to the oxide semiconductor film,specifically, a film containing at least one selected from theconstituent elements (component elements) of the oxide semiconductorfilm is preferably used. Such a material can be fit well to the oxidesemiconductor film, and therefore, provision of the metal oxide filmswith the oxide semiconductor film provided therebetween enables aninterface between the metal oxide film and the oxide semiconductor filmto be kept well. That is, by providing the metal oxide film using theabove-described material(s) as an insulating film which is in contactwith the oxide semiconductor film, accumulation of hydrogen ions in theinterface between the metal oxide film and the oxide semiconductor filmand in the vicinity thereof can be suppressed or prevented. Accordingly,as compared to the case where insulating films each formed of adifferent component/different components from the oxide semiconductorfilm, such as silicon oxide films are provided with the oxidesemiconductor film provided therebetween, the hydrogen concentration inthe interface with the oxide semiconductor film, which affects thethreshold voltage of the transistor, can be sufficiently decreased.

A gallium oxide film is preferably used as the metal oxide film. Sincegallium oxide has a wide bandgap (Eg), by providing gallium oxide filmswith the oxide semiconductor film provided therebetween, an energybarrier is formed in the interface between the oxide semiconductor filmand the metal oxide film to prevent carrier transfer in the interface.Consequently, carriers are not transferred from the oxide semiconductorto the metal oxide, but are transferred within the oxide semiconductorfilm. On the other hand, a hydrogen ion passes through the interfacebetween the oxide semiconductor and the metal oxide and is accumulatedin the vicinity of an interface between the metal oxide and theinsulating film. Even when the hydrogen ion is accumulated in thevicinity of the interface with the insulating film, a parasitic channelthrough which carriers can flow is not formed in the metal oxide filmsuch as a gallium oxide film, which results in no effect or a veryslight effect on the threshold voltage of the transistor. The energybarrier in the case where gallium oxide is in contact with aIn—Ga—Zn—O-based material is about 0.8 eV on the conduction band sideand is about 0.9 eV on the valence band side.

One technological idea of a transistor according to one embodiment ofthe present invention is to increase the amount of oxygen contained inat least one of a gate insulating film in contact with an oxidesemiconductor film, the oxide semiconductor film, and the vicinity of aninterface between them by oxygen doping treatment.

In the case where an oxide semiconductor material which contains indiumwhose bonding strength to oxygen is relatively weak is used for theoxide semiconductor film, when the insulating film in contact with theoxide semiconductor film contains a material which has a strongerbonding strength to oxygen, such as silicon, oxygen in the oxidesemiconductor film may be deprived by heat treatment, which may causeformation of oxygen deficiency in the vicinity of the interface with theoxide semiconductor film. However, in a transistor according to oneembodiment of the present invention, the formation of oxygen deficiencycan be suppressed by supplying excessive oxygen to the oxidesemiconductor film.

Here, after the oxygen doping treatment is performed in themanufacturing process of a transistor, the amount of oxygen which isgreater than the stoichimetric proportion, contained in the oxidesemiconductor film or the gate insulating film in contact with the oxidesemiconductor film may be different between layers. It can be consideredthat chemical potential of oxygen is different between the layers wherethe amount of excessive oxygen is different between them, and thedifference in the chemical potential comes to equilibrium or substantialequilibrium by heat treatment or the like in the manufacturing processof the transistor. Therefore, it is more preferable that heat treatmentbe performed after the oxygen doping treatment on the gate insulatingfilm. With the heat treatment after the oxygen doping treatment, oxygensupplied excessively to the gate insulating film can be diffused tosupply a sufficient amount of oxygen to the oxide semiconductor film.Distribution of oxygen in the equilibrium state is considered below.

The equilibrium state at a temperature T at a pressure P refers to thestate in which a Gibbs free energy of the whole of the systems, G is theminimum, which is represented by the following formula (1).

[FORMULA 1]

G(N _(a) ,N _(b) ,N _(c) , . . . ,T,P)=G ⁽¹⁾(N _(a) ,N _(b) ,N _(c) , .. . ,T,P)+G ⁽²⁾(N _(a) ,N _(b) ,N _(c) , . . . ,T,P)+G ⁽³⁾(N _(a) ,N_(b) ,N _(c) , . . . ,T,P)   (1)

In the formula (1), reference symbols G⁽¹⁾, G⁽²⁾, and G⁽³⁾ denote Gibbsfree energies of layers. Reference symbols N_(a), N_(b), and N_(c)denote respective numbers of particles, and reference symbols a, b, andc denote respective particle kinds. The Gibbs free energy changes asrepresented by the following formula (2) when the particle a istransferred from an i layer to a j layer by δN_(a) ^((j)).

[FORMULA  2] $\begin{matrix}{{\delta \; G} = {{{- \frac{\partial G^{(i)}}{\partial N_{a}^{(i)}}}\delta \; N_{a}^{(j)}} + {\frac{\partial G^{(j)}}{\partial N_{a}^{(j)}}\delta \; N_{a}^{(j)}}}} & (2)\end{matrix}$

When δG is 0 in the formula (2), or when the following formula (3) issatisfied, the systems are in the equilibrium state.

[FORMULA  3] $\begin{matrix}{\frac{\partial G^{(i)}}{\partial N_{a}^{(i)}} = \frac{\partial G^{(j)}}{\partial N_{a}^{(j)}}} & (3)\end{matrix}$

The differential of the Gibbs free energy with respect to the number ofparticles corresponds to the chemical potential, and thus the chemicalpotentials of particles are uniform in the layers in the equilibriumstate.

In other words, specifically, when the amount of oxygen contained in thegate insulating film which is in contact with the oxide semiconductorfilm is excessive as compared to the oxide semiconductor film, thechemical potential of oxygen is relatively small in the oxidesemiconductor film and is relatively large in the gate insulating film.

Then, when the temperature of the whole of the systems (e.g., the oxidesemiconductor film and the gate insulating film in contact with theoxide semiconductor film, here) becomes high enough to cause atomdiffusion in the layer(s) and between the layers by heat treatment inthe manufacturing process of the transistor, oxygen is transferred so asto make the chemical potentials uniform. That is, oxygen in the gateinsulating film is transferred to the oxide semiconductor film, wherebythe chemical potential of the gate insulating film is decreased and thechemical potential of the oxide semiconductor film is increased.

In this manner, oxygen supplied excessively to the gate insulating filmby the oxygen doping treatment is diffused to be supplied to the oxidesemiconductor film by the following heat treatment to make the chemicalpotential of the systems to be in the equilibrium state. As describedabove, oxygen supplied to the oxide semiconductor film is bonded to ahydrogen ion to form an OH group, which does not cause deterioration ofthe transistor or can suppress the deterioration. The supply of oxygento an oxide semiconductor film is also effective for compensation of anoxygen defect in the film.

A transistor including an oxide semiconductor film subjected todehydration or dehydrogenation by heat treatment and oxygen dopingtreatment is a transistor having high reliability in which the amount ofchange in threshold voltage of the transistor by the bias-temperaturestress (BT) test can be reduced.

Accordingly, a transistor having stable electric characteristics can bemanufactured.

According to one embodiment of the present invention, a semiconductordevice having a transistor with high electric characteristics andreliability can be manufactured.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A to 1C illustrate one embodiment of a semiconductor device.

FIGS. 2A to 2E illustrate one embodiment of a method for manufacturing asemiconductor device.

FIGS. 3A to 3D illustrate embodiments of a semiconductor device.

FIGS. 4A to 4F illustrate one embodiment of a method for manufacturing asemiconductor device.

FIGS. 5A to 5C illustrate one embodiment of a method for manufacturing asemiconductor device.

FIG. 6 illustrates one embodiment of a semiconductor device.

FIG. 7 illustrates one embodiment of a semiconductor device.

FIG. 8 illustrates one embodiment of a semiconductor device.

FIGS. 9A and 9B illustrate one embodiment of a semiconductor device.

FIGS. 10A and 10B illustrate electronic equipment.

FIGS. 11A to 11F illustrate electronic equipment.

FIGS. 12A to 12C illustrate embodiments of a semiconductor device.

FIGS. 13A to 13D illustrate one embodiment of a semiconductor device.

FIG. 14A is a top view of a plasma apparatus illustrating one embodimentof the present invention; FIG. 14B is a cross-sectional view thereof.

FIG. 15 is a graph showing measurement results with SIMS.

FIGS. 16A and 16B are cross-sectional STEM images.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. The presentinvention is not limited to the following description, and it is easilyunderstood by those skilled in the art that modes and details of thepresent invention can be modified in various ways without departing fromthe purpose and scope of the present invention. Accordingly, the presentinvention is not construed as being limited to the description of theembodiments and example included herein. The ordinal numbers such as“first” and “second” in this specification are used for convenience anddo not denote the order of steps or the stacking order of layers. Inaddition, the ordinal numbers in this specification do not denoteparticular names which specify the present invention.

Embodiment 1

In Embodiment 1, one embodiment of a semiconductor device and oneembodiment of a method for manufacturing the semiconductor device willbe described using FIGS. 1A to 1C, FIGS. 2A to 2E, and FIGS. 3A to 3D.In this embodiment, a transistor including an oxide semiconductor filmis described as an example of the semiconductor device.

FIGS. 1A to 1C are a plan view and cross-sectional views of abottom-gate transistor described as an example of a semiconductordevice. FIG. 1A is the plan view; FIGS. 1B and 1C are thecross-sectional views along line A-B and line C-D in FIG. 1A,respectively. A gate insulating film 402 is omitted in FIG. 1A.

A transistor 410 shown in FIGS. 1A to 1C includes, over a substrate 400having an insulating surface, a gate electrode layer 401, the gateinsulating film 402, an oxide semiconductor film 403, a source electrodelayer 405 a, and a drain electrode layer 405 b.

In a process for manufacturing the transistor 410, oxygen dopingtreatment is performed on the gate insulating film 402 and heattreatment for dehydration or dehydrogenation is performed on the oxidesemiconductor film 403.

The oxygen doping treatment is addition of an oxygen radical or anoxygen atom or an oxygen ion to a top surface and the bulk of the gateinsulating film. In particular, addition of an oxygen radical or anoxygen atom or an oxygen ion to the top surface and the bulk of the gateinsulating film, with oxygen plasma is also called oxygen plasma dopingtreatment. The substrate over which the gate insulating film is formedis preferably biased while the oxygen plasma doping treatment.

An insulator may be provided over the transistor 410. In order toelectrically connect the source electrode layer 405 a or the drainelectrode layer 405 b to a wiring, an opening may be formed in the gateinsulating film 402 or the like. A second gate electrode may be providedabove the oxide semiconductor film 403. The oxide semiconductor film 403is preferably processed into an island shape but is not necessarilyprocessed into the shape.

FIGS. 2A to 2E illustrate an example of a method for manufacturing thetransistor 410.

First, a conductive film is formed over the substrate 400 having aninsulating surface, and then, subjected to a first photolithographystep, so that the gate electrode layer 401 is formed. Note that a resistmask may be formed by an inkjet method. Formation of the resist mask byan inkjet method needs no photomask; thus, manufacturing cost can bereduced.

There is no particular limitation on a substrate that can be used as thesubstrate 400 having an insulating surface as long as it has heatresistance enough to withstand heat treatment performed later. Forexample, a glass substrate of barium borosilicate glass,aluminoborosilicate glass, or the like, a ceramic substrate, a quartzsubstrate, or a sapphire substrate can be used. A single crystalsemiconductor substrate or a polycrystalline semiconductor substrate ofsilicon, carbon silicon, or the like; a compound semiconductor substrateof silicon germanium or the like; an SOI substrate; or the like can beused as the substrate 400, or the substrate provided with asemiconductor element can be used as the substrate 400.

Further, a flexible substrate may be used as the substrate 400. In thecase where a flexible substrate is used, a transistor including an oxidesemiconductor film may be directly formed over the flexible substrate,or alternatively, a transistor including an oxide semiconductor film maybe formed over another substrate and separated from another substrate tobe transferred to the flexible substrate. In order to separate thetransistor from another substrate and transfer the transistor to theflexible substrate, a separation layer may be provided between thesubstrate and the transistor including the oxide semiconductor film.

An insulating film serving as a base film may be provided between thesubstrate 400 and the gate electrode layer 401. The base film preventsdiffusion of an impurity element from the substrate 400, and can beformed with a single-layer structure or a multi-layer structure usingone or more of a silicon nitride film, a silicon oxide film, a siliconnitride oxide film, and a silicon oxynitride film.

The gate electrode layer 401 can be formed with a single-layer structureor a multi-layer structure using a metal material such as molybdenum,titanium, tantalum, tungsten, aluminum, copper, neodymium, or scandium,and/or an alloy material which contains any of these materials as a maincomponent by a plasma CVD method, a sputtering method, or the like.

Next, the gate insulating film 402 is formed over the gate electrodelayer 401 (see FIG. 2A). The gate insulating film 402 can be formed witha single-layer structure or a multi-layer structure using silicon oxide,silicon nitride, silicon oxynitride, silicon nitride oxide, aluminumoxide, aluminum nitride, aluminum oxynitride, aluminum nitride oxide,hafnium oxide, and/or gallium oxide, and/or a combination thereof by aplasma CVD method, a sputtering method, or the like.

It is far preferable that an insulating material containing acomponent/components similar to the oxide semiconductor film formedlater be used for the gate insulating film 402. This is because such amaterial can be fit well to the oxide semiconductor film, and therefore,this use for the gate insulating film 402 enables a state of aninterface between the gate insulating film 402 and the oxidesemiconductor film to be kept well. To contain the component(s) similarto the oxide semiconductor film means to contain at least one selectedfrom a constituent element or constituent elements of the oxidesemiconductor film. For example, in the case where the oxidesemiconductor film is formed using an In—Ga—Zn-based oxide semiconductormaterial, gallium oxide can be given as an example of the insulatingmaterial containing the component(s) similar to the oxide semiconductorfilm.

As a far preferable example of a multi-layer structure for the gateinsulating film 402, a multi-layer structure of a film (hereinafterreferred to as a film a) containing the insulating material containingthe component(s) similar to the oxide semiconductor film and a film(hereinafter referred to as a film b) containing a material/materialsdifferent from the component material(s) of the film a can be given.This is because with a structure in which the film a and the film b arestacked with the oxide semiconductor film side in order, electricalcharge is preferentially trapped by a charge trapping center in aninterface between the films a and b (as compared to an interface betweenthe oxide semiconductor film and the film a), so that charge trapping inthe interface with the oxide semiconductor film can be sufficientlysuppressed, leading to improvement in the reliability of a semiconductordevice.

A transistor 460 in which a gate insulating film has a multi-layerstructure is shown in FIG. 3B. In the transistor 460, a first gateinsulating film 402 a and a second gate insulating film 402 b arestacked on the gate electrode layer 401, and the oxide semiconductorfilm 403 is formed over the second gate insulating film 402 b. In thetransistor 460, the second gate insulating film 402 b which is incontact with the oxide semiconductor film 403 is the film (film a)containing the insulating material containing the component(s) similarto the oxide semiconductor film 403, the first gate insulating film 402a below the second gate insulating film 402 b is the film (film b)containing a material/materials different from the component material(s)of the second gate insulating film 402 b.

For example, in the case where an In—Ga—Zn-based oxide semiconductorfilm is used as the oxide semiconductor film 403, a gallium oxide filmcan be used as the second gate insulating film 402 b and a silicon oxidefilm can be used as the first gate insulating film 402 a. Further, it ispreferable that a film containing an insulating material containing acomponent/components similar to the oxide semiconductor film be used asan insulating film 407 on and in contact with the oxide semiconductorfilm 403. With the films containing the insulating materials containingthe components similar to the oxide semiconductor film, which areprovided above and below and in contact with the oxide semiconductorfilm 403, the oxide semiconductor film 403 can be surrounded. With astructure in which the films (films a) containing the insulatingmaterials containing the components similar to the oxide semiconductorfilm are provided above and below and in contact with the oxidesemiconductor film 403 and the film (film b) containing thematerial/materials different from the component material(s) of the filmsa is provided outside of the films a, electrical charge can bepreferentially trapped by a charge trapping center in an interfacebetween the film a and the film b above and/or below the oxidesemiconductor film 403, so that charge trapping in an interface with theoxide semiconductor film can be sufficiently suppressed moreeffectively, leading to improvement in the reliability of asemiconductor device.

For the method for manufacturing the gate insulating film 402, ahigh-density plasma CVD method using microwaves (e.g., with a frequencyof 2.45 GHz) is preferably employed because an insulating layer which isdense and can have high breakdown voltage and high quality. This isbecause when the highly purified oxide semiconductor is closely incontact with the high-quality gate insulating film, the interface statedensity can be reduced and interface properties can be favorable.

Further, an insulating layer may be formed, whose film quality andinterface characteristics with the oxide semiconductor are improved byheat treatment which is performed after film formation. In either case,any film can be used as long as film quality as a gate insulating filmis high, interface state density with an oxide semiconductor isdecreased, and a favorable interface can be formed.

Next, oxygen doping treatment is performed on the gate insulating film402. By the oxygen doping treatment on the gate insulating film 402,oxygen 421 is supplied to the gate insulating film 402, so that oxygenis contained in the gate insulating film 402 and/or the vicinity of theinterface (see FIG. 2B). In that case, the amount of oxygen contained ismade to greater than the stoichiometric proportion of the gateinsulating film 402, preferably to greater than the stoichiometricproportion and less than four times as much as that of thestoichiometric proportion thereof, far preferably to greater than thestoichiometric proportion and less than double of the stoichiometricproportion thereof. It can be alternatively said that the amount ofoxygen contained is made to greater than Y, where the amount of oxygencontained in a material of the gate insulating film in the case wherethe material is a single crystal is denoted by Y, preferably to greaterthan Y and less than 4 Y, far preferably to greater than Y and less than2Y. It can be further alternatively said that the amount of oxygencontained is made to greater than Z, where the amount of oxygencontained in a gate insulating film which is subjected to no oxygendoping treatment is denoted by Z, preferably to greater than Z and lessthan 4Z, far preferably to greater than Z and less than 2Z. The oxygen421 for doping contains an oxygen radical, an oxygen atom, and/or anoxygen ion.

For example, in the case of using an oxide insulating film thecomposition of which is represented by GaO_(x) (x>0), since thestoichiometric proportion of gallium oxide is Ga:O=1:1.5, an oxideinsulating film including an oxygen-excessive region where x is greaterthan 1.5 and less than 6 is formed. For example, in the case of using anoxide insulating film the composition of which is represented by SiO_(x)(x>0), since the stoichiometric proportion of silicon oxide is Si:O=1:2,an oxide insulating film including an oxygen-excessive region where x isgreater than 2 and less than 8 is formed. Such an oxygen-excessiveregion may exist in a part of the gate insulating film (including itsinterface). In this manner, the amount of oxygen is made to greater thanthat of hydrogen in the gate insulating film.

In the oxide insulating film which can be used as the gate insulatingfilm, oxygen is one of main component materials. Therefore, it isdifficult to estimate the oxygen concentration of the oxide insulatingfilm accurately with Secondary Ion Mass Spectroscopy (SIMS) or the like.That is, it is difficult to judge whether oxygen is intentionally addedto the oxide insulating film or not.

Isotopes such as ¹⁷O or ¹⁸O exist in oxygen, and it is know that theexistence proportions of them in nature are about 0.037% and about0.204% of the whole oxygen atoms. Therefore, the concentration of suchan isotope in the oxide insulating film can be estimated by SIMS or thelike, and the measurement of such a concentration enables the oxygenconcentration in the oxide insulating film to be estimated accurately.Thus, by measuring the concentration, whether oxygen is intentionallyadded to the oxide insulating film or not may be judged.

For example, with respect to the concentration of ¹⁸O, a concentrationof the isotope of oxygen D1(¹⁸O) in an oxygen-added region and aconcentration of the isotope of oxygen D2(¹⁸O) in a no-oxygen-addedregion have a relationship represented by D1(¹⁸O)>D2(¹⁸O).

Oxygen for the doping may be supplied from a radical generatingapparatus with use of a gas including oxygen or from an ozone generatingapparatus. More specifically, for example, the oxygen 421 can begenerated with an apparatus for etching treatment on a semiconductordevice, an apparatus for ashing on a mask, or the like to process thegate insulating film 402.

In addition, heat treatment (at 150° C. to 470° C.) may be performed onthe gate insulating film 402 which has been subjected to the oxygendoping treatment. By the heat treatment, water or hydroxide generated byreaction between the oxygen 421 and the gate insulating film 402 can beremoved from the gate insulating film 402. The heat treatment may beperformed under an atmosphere of nitrogen, oxygen, an ultra dry air (themoisture amount is less than or equal to 20 ppm (−55° C. by conversioninto a dew point), preferably less than or equal to 1 ppm, farpreferably less than or equal to 10 ppb, in the measurement with the useof a dew point meter of a cavity ring down laser spectroscopy (CRDS)system), or a rare gas (argon, helium, or the like). The atmosphere ofnitrogen, oxygen, the ultra dry air, or the rare gas is preferablyhighly purified without containing water, hydrogen, or the like.

In order that hydrogen, a hydroxyl group, and moisture are contained aslittle as possible in the gate insulating film 402 and the oxidesemiconductor film provided over the gate insulating film 402, it ispreferable that the substrate 400 over which the gate electrode layer401 is formed or the substrate 400 which has been subjected to themanufacturing process up to and including the step for forming the gateinsulating film 402 be preheated in a preheating chamber of a sputteringapparatus as pretreatment for the formation of the oxide semiconductorfilm, so that impurities such as hydrogen and moisture adsorbed to thesubstrate 400 are eliminated and removed. As an exhaustion unit providedin the preheating chamber, a cryopump is preferable. This preheatingtreatment is not necessarily performed. Further, this preheating may beperformed on the substrate 400 which has been subjected to themanufacturing process up to and including the step for forming thesource electrode layer 405 a and the drain electrode layer 405 b, beforethe formation of the insulating film 407.

Next, over the gate insulating film 402, an oxide semiconductor filmwith a thickness of greater than or equal to 2 nm and less than or equalto 200 nm, preferably greater than or equal to 5 nm and less than orequal to 30 nm is fondled.

As an oxide semiconductor used for the oxide semiconductor film, any ofthe following oxide semiconductors can be used: a four-component metaloxide such as an In—Sn—Ga—Zn—O-based oxide semiconductor; athree-component metal oxide such as an In—Ga—Zn—O-based oxidesemiconductor, an In—Sn—Zn—O-based oxide semiconductor, anIn—Al—Zn—O-based oxide semiconductor, a Sn—Ga—Zn—O-based oxidesemiconductor, an Al—Ga—Zn—O-based oxide semiconductor, or aSn—Al—Zn—O-based oxide semiconductor; a two-component metal oxide suchas an In—Zn—O-based oxide semiconductor, a Sn—Zn—O-based oxidesemiconductor, an Al—Zn—O-based oxide semiconductor, a Zn—Mg—O-basedoxide semiconductor, a Sn—Mg—O-based oxide semiconductor, anIn—Mg—O-based oxide semiconductor, or an In—Ga—O-based oxidesemiconductor; a an In—O-based oxide semiconductor; a Sn—O-based oxidesemiconductor; a Zn—O-based oxide semiconductor; and the like. Further,SiO₂ may be contained in the above oxide semiconductor. Note that here,for example, the In—Ga—Zn—O-based oxide semiconductor means an oxidefilm containing indium (In), gallium (Ga), and zinc (Zn) and there is noparticular limitation on the stoichiometric proportion. TheIn—Ga—Zn—O-based oxide semiconductor may contain an element other thanIn, Ga, and Zn.

As the oxide semiconductor film, a thin film of a material representedby the chemical formula, InMO₃(ZnO)_(m) (m>0), can be used. Here, Mrepresents one or more metal elements selected from Ga, Al, Mn, and Co.For example, M may be Ga, Ga and Al, Ga and Mn, Ga and Co, or the like.

For the oxide semiconductor film, an oxide semiconductor containingindium, an oxide semiconductor containing indium and gallium, or thelike can be preferably used.

In this embodiment, the oxide semiconductor film is formed by asputtering method using an In—Ga—Zn—O-based oxide semiconductor target.The oxide semiconductor film can be formed by a sputtering method in arare gas (typically, argon) atmosphere, an oxygen atmosphere, or a mixedatmosphere of a rare gas and oxygen.

A target used for formation of the oxide semiconductor film by asputtering method is, for example, an oxide target containing In₂O₃,Ga₂O₃, and ZnO at a composition ratio of 1:1:1 [molar ratio], so that anIn—Ga—Zn—O film is formed. Without limitation on the material and thecomposition of the target, for example, an oxide target having acomposition ratio of In₂O₃:Ga₂O₃:ZnO=1:1:2 [molar ratio] may be used.

Furthermore, the filling rate of the oxide target is 90% to 100%,preferably 95% to 99.9%. With use of the metal oxide target with such ahigh filling rate, a dense oxide semiconductor film can be formed.

It is preferable that a high-purity gas with an impurity such ashydrogen, water, hydroxyl, or hydride removed be used as a sputteringgas for forming the oxide semiconductor film.

The substrate is held in a deposition chamber kept under reducedpressure, and the substrate temperature is set to temperatures higherthan or equal to 100° C. and lower than or equal to 600° C., preferablyhigher than or equal to 200° C. and lower than or equal to 400° C. Byforming the oxide semiconductor film while heating the substrate, theconcentration of the impurity included in the oxide semiconductor filmcan be reduced. In addition, damage by sputtering can be suppressed.Then, residual moisture in the deposition chamber is removed, asputtering gas from which hydrogen and moisture are removed isintroduced, and the above-described target is used, so that the oxidesemiconductor film is formed over the substrate 400. In order to removemoisture remaining in the deposition chamber, an entrapment vacuum pumpsuch as a cryopump, an ion pump, or a titanium sublimation pump ispreferably used. As an exhaustion unit, a turbo molecular pump to whicha cold trap is added may be used. In the deposition chamber which isevacuated with the cryopump, for example, a hydrogen atom, a compoundcontaining a hydrogen atom, such as water, (more preferably, also acompound containing a carbon atom), and the like are removed, wherebythe concentration of an impurity in the oxide semiconductor film formedin the deposition chamber can be reduced.

As one example of the film formation condition, the following isemployed: the distance between the substrate and the target is 100 mm,the pressure is 0.6 Pa, the direct-current (DC) power is 0.5 kW, and theatmosphere is an oxygen atmosphere (the proportion of the oxygen flowrate is 100%). A pulsed direct-current power source is preferably usedsince powder substances (also referred to as particles or dust) that aregenerated in deposition can be reduced and the film thickness comes tobe uniform.

Next, the oxide semiconductor film is processed into an island-shapedoxide semiconductor film 441 by a second photolithography step (see FIG.2C). A resist mask used for forming the island-shaped oxidesemiconductor film 441 may be formed by an inkjet method. Formation ofthe resist mask by an inkjet method needs no photomask; thus,manufacturing cost can be reduced.

In the case where a contact hole is formed in the gate insulating film402, a step of forming the contact hole can be performed at the sametime as processing of the oxide semiconductor film.

Note that the etching of the oxide semiconductor film may be dryetching, wet etching, or both dry etching and wet etching. As an etchantused for wet etching of the oxide semiconductor film, for example, amixed solution of phosphoric acid, acetic acid, and nitric acid, or thelike can be used. As the etchant, ITO07N (produced by KANTO CHEMICALCO., INC.) may be used as well.

Next, the oxide semiconductor film 441 is subjected to heat treatment.With this heat treatment, excessive hydrogen (including water and ahydroxyl group) can be removed (dehydration or dehydrogenation), thestructure of the oxide semiconductor film can be improved, and defectlevels in an energy gap can be reduced. The temperature of the heattreatment is higher than or equal to 250° C. and lower than or equal to750° C., or higher than or equal to 400° C. and less than the strainpoint of the substrate. In this embodiment, the substrate is put in anelectric furnace which is a kind of heat treatment apparatus and theoxide semiconductor film is subjected to heat treatment at 450° C. forone hour in a nitrogen atmosphere, and then, water and hydrogen areprevented from being mixed into the oxide semiconductor film bypreventing the substrate from being exposed to the air; thus, the oxidesemiconductor film 403 is obtained (see FIG. 2D).

Note that the heat treatment apparatus is not limited to the electricfurnace, and an apparatus for heating an object by heat conduction orheat radiation from a heater such as a resistance heater may be used.For example, an RTA (rapid thermal anneal) apparatus such as a GRTA (gasrapid thermal anneal) apparatus or an LRTA (lamp rapid thermal anneal)apparatus can be used. An LRTA apparatus is an apparatus for heating anobject by radiation of light (an electromagnetic wave) emitted from alamp such as a halogen lamp, a metal halide lamp, a xenon arc lamp, acarbon arc lamp, a high pressure sodium lamp, or a high pressure mercurylamp. A GRTA apparatus is an apparatus for performing heat treatmentusing a high-temperature gas. As the high temperature gas, an inert gaswhich does not react with an object by heat treatment, such as nitrogenor a rare gas like argon, is used.

For example, as the heat treatment, GRTA may be performed, in which thesubstrate is moved into an inert gas heated at a high temperature of650° C. to 700° C., and heated for several minutes, and then thesubstrate is moved out of the inert gas.

In the heat treatment, it is preferable that an impurity such as water,hydrogen, and the like be not contained in nitrogen or the rare gas suchas helium, neon, or argon. The purity of nitrogen or the rare gas suchas helium, neon, or argon which is introduced into the heat treatmentapparatus is set to preferably 6N (99.9999%) or higher, far preferably7N (99.99999%) or higher (that is, the impurity concentration ispreferably 1 ppm or lower, far preferably 0.1 ppm or lower).

In addition, after the oxide semiconductor film is heated by the heattreatment, a high-purity oxygen gas, a high-purity N₂O gas, or ultra dryair (the moisture amount is less than or equal to 20 ppm (−55° C. byconversion into a dew point), preferably less than or equal to 1 ppm,far preferably less than or equal to 10 ppb, in the measurement with theuse of a dew point meter of a cavity ring down laser spectroscopy (CRDS)system) may be introduced into the same furnace. It is preferable thatwater, hydrogen, or the like be not contained in the oxygen gas or theN₂O gas. The purity of the oxygen gas or the N₂O gas which is introducedinto the heat treatment apparatus is preferably 6N or more, farpreferably 7N or more (i.e., the impurity concentration in the oxygengas or the N₂O gas is preferably 1 ppm or lower, far preferably 0.1 ppmor lower). The oxygen gas or the N₂O gas acts to supply oxygen that is amain component of the oxide semiconductor and that is reduced by thestep for removing an impurity for the dehydration or dehydrogenation, sothat the oxide semiconductor film can be a high-purified, electricallyi-type (intrinsic) oxide semiconductor film.

The heat treatment can also be performed on the oxide semiconductor filmbefore the oxide semiconductor film is processed into the island-shapedoxide semiconductor film. In that case, after the heat treatment, thesubstrate is taken out of the heating apparatus and a photolithographystep is performed on the oxide semiconductor film. The heat treatmentmay be performed after a source electrode layer and a drain electrodelayer are formed over the island-shaped oxide semiconductor film as longas the oxide semiconductor film is formed before that heat treatment.

Since the amount of oxygen contained in the gate insulating film 402 isexcessive because of the oxygen doping treatment performed on the gateinsulating film 402, oxygen transfer from the oxide semiconductor film403 stacked on and in contact with the gate insulating film 402 can besuppressed. Further, since the oxide semiconductor film 403 is stackedon and in contact with the gate insulating film 402 subjected to theoxygen doping treatment, oxygen can be supplied from the gate insulatingfilm 402 (bulk thereof and/or interface thereof) to the oxidesemiconductor film 403. The oxygen supply from the gate insulating film402 to the oxide semiconductor film 403 is further promoted by heattreatment on a state where the gate insulating film 402 subjected to theoxygen doping treatment is in contact with the oxide semiconductor film403. That heat treatment can also serve as heat treatment fordehydration or dehydrogenation.

The oxygen 421 added to the gate insulating film 402 and supplied to theoxide semiconductor film 403 preferably has at least partly a danglingbond of oxygen in the oxide semiconductor. This is because the danglingbond can be bonded with hydrogen left in the film to immobilize hydrogen(make hydrogen an immovable ion).

Through the above steps, the oxide semiconductor film 403 is highlypurified and is made electrically i-type (intrinsic).

Next, a conductive film for forming a source electrode layer and a drainelectrode layer (including a wiring formed of the same layer as thesource electrode layer and the drain electrode layer) is formed over thegate insulating film 402 and the oxide semiconductor film 403. As theconductive film serving as the source electrode layer and the drainelectrode layer, for example, a metal film including an element selectedfrom Al, Cr, Cu, Ta, Ti, Mo, and W, or a metal nitride film includingany of the above elements as its component (e.g., a titanium nitridefilm, a molybdenum nitride film, or a tungsten nitride film) can beused. A film of a high-melting-point metal such as Ti, Mo, or W or ametal nitride film thereof (e.g., a titanium nitride film, a molybdenumnitride film, or a tungsten nitride film) may be provided over or/andbelow the metal film such as an Al film or a Cu film to form theconductive film serving as the source electrode layer and the drainelectrode layer. Alternatively, the conductive film used for the sourceelectrode layer and the drain electrode layer may be formed using aconductive metal oxide. As the conductive metal oxide, indium oxide(In₂O₃), tin oxide (SnO₂), zinc oxide (ZnO), indium oxide-tin oxidemixed oxide (In₂O₃—SnO₂; abbreviated to ITO), indium oxide-zinc oxidemixed oxide (In₂O₃—ZnO), or any of these metal oxide materials in whichsilicon or silicon oxide is contained can be used.

A resist mask is formed over the conductive film by a thirdphotolithography step, and is selectively etched, so that the sourceelectrode layer 405 a and the drain electrode layer 405 b are formed.Then, the resist mask is removed.

In order to reduce the number of photomasks used in a photolithographystep and reduce the number of photolithography steps, an etching stepmay be performed with the use of a multi-tone mask which is alight-exposure mask through which light is transmitted to have aplurality of intensities. A resist mask formed with the use of amulti-tone mask has a plurality of thicknesses and further can bechanged in shape by etching; therefore, the resist mask can be used in aplurality of etching steps for processing into different patterns.Therefore, a resist mask corresponding to at least two kinds ofdifferent patterns can be formed with one multi-tone mask. Thus, thenumber of photomasks can be reduced and the number of photolithographysteps can be also reduced accordingly, whereby simplification of amanufacturing process can be realized.

It is preferable that etching conditions be optimized so as not to etchand cut the oxide semiconductor film 403 when the conductive film isetched. However, it is difficult to obtain etching conditions in whichonly the conductive film is etched and the oxide semiconductor film 403is not etched at all. In some cases, part of the oxide semiconductorfilm 441 is etched off through the etching of the conductive film, sothat an oxide semiconductor film having a groove portion (a depressedportion) is formed.

In this embodiment, a Ti film is used as the conductive film and anIn—Ga—Zn—O-based oxide semiconductor is used as the oxide semiconductorfilm 403, and therefore, ammonium hydrogen peroxide (a mixture ofammonia, water, and hydrogen peroxide) is used as an etchant.

The number of carriers in the highly purified oxide semiconductor film403 is significantly small (close to zero).

Through the above process, the transistor 410 is formed (see FIG. 2E).The transistor 410 is a transistor including the oxide semiconductorfilm 403 which is highly purified and from which an impurity such ashydrogen, moisture, a hydroxyl group, or hydride (also referred to as ahydrogen compound) is removed. Therefore, variation in the electriccharacteristics of the transistor 410 is suppressed and the transistor410 is electrically stable.

Further, as shown in FIG. 3A, a transistor 440 in which an insulatingfilm 407 and an insulating film 409 are provided over the oxidesemiconductor film 403 and the source electrode layer 405 a and thedrain electrode layer 405 b can be formed.

The insulating film 407 can be formed to a thickness of at least 1 nm bya method by which an impurity such as water and hydrogen does not enterthe insulating film 407, such as a sputtering method, as appropriate.When hydrogen is contained in the insulating film 407, entry of hydrogeninto the oxide semiconductor film or extraction of oxygen from the oxidesemiconductor film by hydrogen is caused; thus, the resistance of a backchannel of the oxide semiconductor film might become low (theconductivity of the same might be n-type) and a parasitic channel mightbe formed. Therefore, it is important that a film formation method inwhich hydrogen is not used is employed in order to form the insulatingfilm 407 containing as little hydrogen as possible.

As the insulating film 407, an inorganic insulating film such as asilicon oxide film, a silicon oxynitride film, an aluminum oxide film,an aluminum oxynitride film, or a gallium oxide film can be typicallyused.

In this embodiment, a 200-nm-thick gallium oxide film is deposited asthe insulating film 407 by a sputtering method.

It is far preferable that an insulating material containing acomponent/components similar to the oxide semiconductor film 403 be usedfor the insulating film 407, like the gate insulating film 402. This isbecause such a material can be fit well to the oxide semiconductor film,and therefore, this use for the insulating film 407 enables a state ofan interface between the insulating film and the oxide semiconductorfilm to be kept well. For example, in the case where the oxidesemiconductor film is formed using an In—Ga—Zn—O-based oxidesemiconductor material, gallium oxide can be given as an example of theinsulating material containing the component(s) similar to the oxidesemiconductor film 403.

As a far preferable example of a multi-layer structure for theinsulating film 407, a multi-layer structure of a film (hereinafterreferred to as a film a) containing the insulating material containingthe component(s) similar to the oxide semiconductor film and a film(hereinafter referred to as a film b) containing a material/materialsdifferent from the component material(s) of the film a can be given.This is because with a structure in which the film a and the film b arestacked on the oxide semiconductor film side in order, electrical chargeis preferentially trapped by a charge trapping center in an interfacebetween the films a and b (as compared to an interface between the oxidesemiconductor film and the film a), so that charge trapping in theinterface with the oxide semiconductor film can be sufficientlysuppressed, leading to improvement in the reliability of a semiconductordevice.

For example, a multi-layer in which a gallium oxide film and a siliconoxide film are stacked on the oxide semiconductor film 403 side, or amulti-layer in which a gallium oxide film and a silicon nitride film arestacked on the oxide semiconductor film 403 side can be preferably usedas the insulating film 407.

The substrate temperature at the time of the formation of the siliconoxide film may be higher than or equal to room temperature and lowerthan or equal to 300° C.; in this embodiment, the substrate temperatureis 100° C. as an example. The silicon oxide film can be formed by asputtering method in a rare gas (typically, argon) atmosphere, an oxygenatmosphere, or a mixed atmosphere of a rare gas and oxygen. As a target,a silicon oxide target or a silicon target can be used. For example, thesilicon oxide film can be formed using a silicon target by a sputteringmethod in an atmosphere containing oxygen.

In order to remove residual moisture from the deposition chamber of theinsulating film 407 in a manner similar to that of the formation of theoxide semiconductor film, an entrapment vacuum pump (such as a cryopump)is preferably used. When the insulating film 407 is deposited in thedeposition chamber evacuated using a cryopump, the impurityconcentration of the insulating film 407 can be reduced. As anevacuation unit for removing moisture remaining in the depositionchamber of the insulating film 407, a turbo molecular pump provided witha cold trap may be used.

It is preferable that a high-purity gas from which an impurity such ashydrogen, water, hydroxyl group, or hydride be removed be used as asputtering gas used for formation of the insulating film 407.

It is preferable to perform heat treatment after the formation of theinsulating film 407. The heat treatment is performed at a temperaturehigher than or equal to 250° C. and lower than or equal to 700° C.,preferably higher than or equal to 450° C. and lower than or equal to600° C. or less than a strain point of the substrate.

The heat treatment may be performed under an atmosphere of nitrogen,oxygen, an ultra dry air (the moisture amount is less than or equal to20 ppm (−55° C. by conversion into a dew point), preferably less than orequal to 1 ppm, far preferably less than or equal to 10 ppb, in themeasurement with the use of a dew point meter of a cavity ring downlaser spectroscopy (CRDS) system), or a rare gas (argon, helium, or thelike). The atmosphere of nitrogen, oxygen, the ultra dry air, or therare gas preferably contains water, hydrogen, or the like as less aspossible. The purity of nitrogen, oxygen, or the rare gas which isintroduced into the heat treatment apparatus is set to preferably 6N(99.9999%) or higher, far preferably 7N (99.99999%) or higher (that is,the impurity concentration is preferably 1 ppm or lower, far preferably0.1 ppm or lower).

In the case where the insulating film 407 contains oxygen and the heattreatment is performed on the state where the oxide semiconductor filmis in contact with the insulating film 407, oxygen can be furthersupplied to the oxide semiconductor film from the insulating film 407containing oxygen.

It is preferable to form the insulating film 409 over the insulatingfilm 407, as a protective insulating film for blocking to prevententrance of an impurity such as moisture or hydrogen into the oxidesemiconductor film 403 and to prevent discharge of oxygen from the gateinsulating film 402, the oxide semiconductor film 403, the insulatingfilm 407, and an interface thereof. As the insulating film 409, it ispreferable to use an inorganic insulating film such as a silicon nitridefilm, an aluminum oxide film, or the like. For example, a siliconnitride film is formed by an RF sputtering method. An RF sputteringmethod is preferable as a method for forming the insulating film 409because of its high productivity.

Heat treatment may be performed after the insulating film 409 is formed.For example, the heat treatment may be performed at a temperature higherthan or equal to 100° C. and lower than or equal to 200° C. in the airfor 1 hour to 30 hours. This heat treatment may be performed at a fixedheating temperature; alternatively, the following change in the heatingtemperature may be conducted plural times: the heating temperature isincreased from room temperature to a temperature higher than or equal to100° C. and lower than or equal to 200° C. and then decreased to roomtemperature.

Other structures of transistors including an oxygen-excessive regionsubjected to oxygen doping treatment are shown in FIGS. 3C and 3D.

A transistor 420 illustrated in FIG. 3C is one of bottom-gatetransistors referred to as a channel-protective (channel-stop)transistor and is also referred to as an inverted-staggered transistor.

The transistor 420 includes, over the substrate 400 having an insulatingsurface, the gate electrode layer 401, the gate insulating film 402, theoxide semiconductor film 403, an insulating film 427 functioning as achannel protective layer covering a channel formation region of theoxide semiconductor film 403, the source electrode layer 405 a, and thedrain electrode layer 405 b. The insulating film 409 is formed so as tocover the transistor 420.

A transistor 430 shown in FIG. 3D is a bottom-gate transistor andincludes, over the substrate 400 having an insulating surface, the gateelectrode layer 401, the gate insulating film 402, the source electrodelayer 405 a, the drain electrode layer 405 b, and the oxidesemiconductor film 403. The insulating film 407 which covers thetransistor 430 and is in contact with the oxide semiconductor film 403is provided. The insulating film 409 is provided over the insulatingfilm 407.

In the transistor 430, the gate insulating film 402 is provided on andin contact with the substrate 400 and the gate electrode 401, and thesource electrode 405 a and the drain electrode 405 b are provided on andin contact with the gate insulating film 402. Further, the oxidesemiconductor film 403 is provided over the gate insulating film 402,the source electrode layer 405 a, and the drain electrode layer 405 b.

In each of the transistors 410, 420, 430, and 440 including thehighly-purified oxide semiconductor film 403 according to thisembodiment, the current in an off state (the off-state current) can besmall.

Such a transistor including an oxide semiconductor film subjected tooxygen doping treatment is a transistor having high reliability in whichthe amount of change in threshold voltage of the transistor by thebias-temperature stress (BT) test can be reduced.

Further, in the transistors 410, 420, 430, and 440 each including theoxide semiconductor film 403, relatively high field-effect mobility canbe obtained, which enables high-speed operation. Consequently, with theabove transistor provided in a pixel portion of a semiconductor devicehaving a display function, high-quality images can be displayed. Inaddition, by using the transistor including the highly purified oxidesemiconductor film, a driver circuit portion and a pixel portion can beformed over one substrate, whereby the number of components of thesemiconductor device can be reduced.

In this manner, a semiconductor device including an oxide semiconductor,which has stable electric characteristics, can be provided. Accordingly,a semiconductor device with high reliability can be provided.

Embodiment 2

In Embodiment 2, another embodiment of a semiconductor device and oneembodiment of a method for manufacturing the semiconductor device willbe described using FIGS. 4A to 4F and 5A to 5C. In this embodiment, atransistor including an oxide semiconductor film will be described as anexample of a semiconductor device. The same portions as and portionshaving functions similar to those described in Embodiment 1 can beformed in a manner similar to that described in Embodiment 1; therefore,description thereof is omitted. In addition, detailed description of thesame portions is omitted.

An example of a method for manufacturing a transistor 450 is shown inFIGS. 4A to 4F and 5A to 5C. In this embodiment, oxygen doping treatmentis performed plural times in a manufacturing process of the transistor450.

First, a conductive film is formed over the substrate 400 having aninsulating surface and is subjected to a first photolithography step toform the gate electrode layer 401.

Next, the gate insulating film 402 is formed over the gate electrodelayer 401 (see FIG. 4A).

Next, oxygen doping treatment is performed on the gate insulating film402. By the oxygen doping treatment on the gate insulating film 402,oxygen 421 a is supplied to the gate insulating film 402, so that oxygenis contained in the gate insulating film 402 and/or the vicinity of theinterfaces (see FIG. 4B). In that case, the amount of oxygen containedis made to greater than the stoichiometric proportion of the gateinsulating film 402, preferably to greater than the stoichiometricproportion and less than four times as much as the stoichiometricproportion thereof, far preferably to greater than the stoichiometricproportion and less than double of the stoichiometric proportionthereof. It can be alternatively said that the amount of oxygencontained is made to greater than Y, where the amount of oxygen of amaterial of the gate insulating film in the case where the material is asingle crystal is denoted by Y, preferably to greater than Y and lessthan 4Y, far preferably to greater than Y and less than 2Y. It can befurther alternatively said that the amount of oxygen contained is madeto greater than Z, where the amount of oxygen contained in a gateinsulating film which is subjected to no oxygen doping treatment isdenoted by Z, preferably to greater than Z and less than 4Z, farpreferably to greater than Z and less than 2Z. The oxygen 421 a fordoping contains an oxygen radical, an oxygen atom, and/or an oxygen ion.

For example, in the case of using an oxide insulating film thecomposition of which is represented by GaO_(x) (x>0), since thestoichiometric proportion of gallium oxide is Ga:O=1:1.5, an oxideinsulating film including an oxygen-excessive region where x is greaterthan 1.5 and less than 6 is formed. For example, in the case of using anoxide insulating film the composition of which is represented by SiO_(x)(x>0), since the stoichiometric proportion of silicon oxide is Si:O=1:2,an oxide insulating film including an oxygen-excessive region where x isgreater than 2 and less than 8 is formed. Such an oxygen-excessiveregion may exist in a part of the gate insulating film (including itsinterface). In this manner, the amount of oxygen is made to greater thanthat of hydrogen in the gate insulating film.

Oxygen for the doping may be supplied from a radical generatingapparatus with use of a gas including oxygen or from an ozone generatingapparatus. More specifically, for example, the oxygen 421 a can begenerated with an apparatus for etching treatment on a semiconductordevice, an apparatus for ashing on a mask, or the like to process thegate insulating film 402.

In addition, heat treatment (at 150° C. to 470° C.) may be performed onthe gate insulating film 402 which has been subjected to the oxygendoping treatment. By the heat treatment, water or hydroxide generated byreaction between the oxygen 421 a and the gate insulating film 402 canbe removed from the gate insulating film 402. The heat treatment may beperformed under an atmosphere of nitrogen, oxygen, an ultra dry air (themoisture amount is less than or equal to 20 ppm (−55° C. by conversioninto a dew point), preferably less than or equal to 1 ppm, farpreferably less than or equal to 10 ppb, in the measurement with the useof a dew point meter of a cavity ring down laser spectroscopy (CRDS)system), or a rare gas (argon, helium, or the like). The atmosphere ofnitrogen, oxygen, the ultra dry air, or the rare gas is preferablyhighly purified without containing water, hydrogen, or the like.

In order that hydrogen, a hydroxyl group, and moisture are contained aslittle as possible in the gate insulating film 402 and the oxidesemiconductor film provided over the gate insulating film 402, it ispreferable that the substrate 400 over which the gate electrode layer401 is formed or the substrate 400 which has been subjected to themanufacturing process up to and including the step for forming the gateinsulating film 402 be preheated in a preheating chamber of a sputteringapparatus as pretreatment for the formation of the oxide semiconductorfilm, so that impurities such as hydrogen and moisture adsorbed to thesubstrate 400 are eliminated and removed. As an exhaustion unit providedin the preheating chamber, a cryopump is preferable. This preheatingtreatment is not necessarily performed. Further, this preheating may beperformed on the substrate 400 which has been subjected to themanufacturing process up to and including the step for forming thesource electrode layer 405 a and the drain electrode layer 405 b, beforethe formation of the insulating film 407.

Next, over the gate insulating film 402, an oxide semiconductor filmwith a thickness of greater than or equal to 2 nm and less than or equalto 200 nm, preferably greater than or equal to 5 nm and less than orequal to 30 nm is formed.

In this embodiment, the oxide semiconductor film is formed by asputtering method using an In—Ga—Zn—O-based oxide semiconductor target.The oxide semiconductor film can be formed by a sputtering method in arare gas (typically, argon) atmosphere, an oxygen atmosphere, or a mixedatmosphere of a rare gas and oxygen.

The target used for formation of the oxide semiconductor film by asputtering method is, for example, an oxide target containing In₂O₃,Ga₂O₃, and ZnO at a composition ratio of 1:1:1 [molar ratio], so that anIn—Ga—Zn—O film can be formed.

It is preferable that a high-purity gas from which an impurity such ashydrogen, water, a hydroxyl group, or hydride is removed be used as asputtering gas used for forming the oxide semiconductor film.

As one example of the film formation condition, the following isemployed: the distance between the substrate and the target is 100 mm,the pressure is 0.6 Pa, the direct-current (DC) power is 0.5 kW, and theatmosphere is an oxygen atmosphere (the proportion of the oxygen flowrate is 100%). Note that a pulsed direct-current power source ispreferably used, in which case powder substances (also referred to asparticles or dust) that are generated in deposition can be reduced andthe film thickness can be uniform.

Next, the oxide semiconductor film is processed into the island-shapedoxide semiconductor film 441 through a second photolithography step (seeFIG. 4C).

Next, the oxide semiconductor film 441 is subjected to heat treatment.With this heat treatment, excessive hydrogen (including water and ahydroxyl group) can be removed (dehydration or dehydrogenation), thestructure of the oxide semiconductor film can be improved, and defectlevels in an energy gap can be reduced. The temperature of the heattreatment is higher than or equal to 250° C. and lower than or equal to750° C., or higher than or equal to 400° C. and lower than the strainpoint of the substrate. In this embodiment, the substrate is put in anelectric furnace which is a kind of heat treatment apparatus and theoxide semiconductor film is subjected to heat treatment at 450° C. forone hour in a nitrogen atmosphere, and then, water and hydrogen areprevented from being mixed into the oxide semiconductor film bypreventing the substrate from being exposed to the air; thus, the oxidesemiconductor film 403 is obtained (see FIG. 4D).

The heat treatment apparatus is not limited to the electric furnace, andan apparatus for heating an object by heat conduction or heat radiationfrom a heater such as a resistance heater may be used.

For example, as the heat treatment, GRTA may be performed, in which thesubstrate is moved into an inert gas heated at a high temperature of650° C. to 700° C., and heated for several minutes, and then thesubstrate is moved out of the inert gas.

The heat treatment can also be performed on the oxide semiconductor filmbefore the oxide semiconductor film is processed into the island-shapedoxide semiconductor film. In that case, after the heat treatment, thesubstrate is taken out of the heating apparatus and a photolithographystep is performed on the oxide semiconductor film. The heat treatmentmay be performed after a source electrode layer and a drain electrodelayer are formed over the island-shaped oxide semiconductor film as longas the oxide semiconductor film is formed before that heat treatment.

Next, oxygen doping treatment is performed on the dehydrated ordehydrogenated oxide semiconductor film 403. By the oxygen dopingtreatment on the oxide semiconductor film 403, oxygen 421 b is suppliedto the oxide semiconductor film 403, so that oxygen is contained in theoxide semiconductor film 403 and/or the vicinity of the interface (seeFIG. 4E). In that case, the amount of oxygen contained is made togreater than the stoichiometric proportion of the oxide semiconductorfilm 403, preferably to greater than the stoichiometric proportion andless than double of the stoichiometric proportion. It can bealternatively said that the amount of oxygen contained is made togreater than Y, where the amount of oxygen contained in a singlecrystalline semiconductor film is denoted by Y, preferably to greaterthan Y and less than 2Y. It can be further alternatively said that theamount of oxygen contained is made to greater than Z, where the amountof oxygen contained in an oxide semiconductor film which is subjected tono oxygen doping treatment is denoted by Z, preferably to greater than Zand less than 2Z. Too much of oxygen content may lead to absorption ofhydrogen into the oxide semiconductor film 403, like a hydrogen storingalloy (hydrogen storage alloy). The oxygen 421 b for doping contains anoxygen radical, an oxygen atom, and/or an oxygen ion.

For example, in the case of using a material a single crystal structureof which is represented by InGaO₃(ZnO)_(m) (m>0), the composition of theoxide semiconductor film 403 is represented by InGaZn_(m)O_(x);therefore, in the case where m is 1 (InGaZnO₄), the acceptable x isgreater than 4 and less than 8, and in the case where m is 2(InGaZn₂O₅), the acceptable x is greater than 5 and less than 10. Suchan oxygen-excessive region may exist in a part of the oxidesemiconductor film (including its interface). In this manner, the amountof oxygen is made to greater than that of hydrogen in the oxidesemiconductor film.

In the oxide semiconductor film, oxygen is one of main componentmaterials. Therefore, it is difficult to estimate the oxygenconcentration of the oxide semiconductor film accurately with SecondaryIon Mass Spectrometry (SIMS) or the like. That is, it is difficult tojudge whether oxygen is intentionally added to the oxide semiconductorfilm or not.

Isotopes such as ¹⁷O or ¹⁸O exist in oxygen, and it is know that theexistence proportions of them in nature are about 0.037% and about0.204% of the whole oxygen atoms. Therefore, the concentration of suchan isotope in the oxide semiconductor film can be estimated by SIMS orthe like, and the measurement of such a concentration enables the oxygenconcentration in the oxide semiconductor film to be estimatedaccurately. Thus, by measuring the concentration, whether oxygen isintentionally added to the oxide semiconductor film or not may bejudged.

For example, with respect to the concentration of ¹⁸O, a concentrationof the isotope of oxygen D1(¹⁸O) in an oxygen-added region and aconcentration of the isotope of oxygen D2(¹⁸O) in a no-oxygen-addedregion have a relationship represented by D1(¹⁸O)>D2(¹⁸O).

The oxygen 421 b added to (contained in) the oxide semiconductor filmpreferably has at least partly a dangling bond of oxygen in the oxidesemiconductor. This is because the dangling bond can be bonded withhydrogen left in the film to immobilize hydrogen (make hydrogen animmovable ion).

Oxygen for the doping (an oxygen radical, an oxygen atom, and/or anoxygen ion) may be supplied from a radical generating apparatus with useof a gas including oxygen or from an ozone generating apparatus. Morespecifically, for example, the oxygen 421 b can be generated with anapparatus for etching treatment on a semiconductor device, an apparatusfor ashing on a resist mask, or the like to process the oxidesemiconductor film 403.

It is preferable to electrically bias the substrate in order to addoxygen more preferably.

In addition, heat treatment (at 150° C. to 470° C.) may be performed onthe oxide semiconductor film 403 which has been subjected to the oxygendoping treatment. By the heat treatment, water or hydroxide generated byreaction between the oxygen 421 b and the oxide semiconductor film 403can be removed from the oxide semiconductor film 403. The heat treatmentmay be performed under an atmosphere of nitrogen, oxygen, an ultra dryair (the moisture amount is less than or equal to 20 ppm (−55° C. byconversion into a dew point), preferably less than or equal to 1 ppm,far preferably less than or equal to 10 ppb, in the measurement with theuse of a dew point meter of a cavity ring down laser spectroscopy (CRDS)system), or a rare gas (argon, helium, or the like). The atmosphere ofnitrogen, oxygen, the ultra dry air, or the rare gas is preferablyhighly purified without containing water, hydrogen, or the like.

Through the above steps, the oxide semiconductor film 403 is highlypurified and is made electrically i-type (intrinsic).

Since the amount of oxygen contained in the gate insulating film 402 isexcessive because of the oxygen doping treatment performed on the gateinsulating film 402, oxygen transfer from the oxide semiconductor film403 stacked on can be suppressed. Further, since the oxide semiconductorfilm 403 is stacked on and in contact with the gate insulating film 402subjected to the oxygen doping treatment, oxygen can be supplied fromthe gate insulating film 402 (bulk thereof and/or interface thereof) tothe gate insulating film 402. The oxygen supply from the gate insulatingfilm 402 to the oxide semiconductor film 403 is further promoted by heattreatment on a state where the gate insulating film 402 subjected to theoxygen doping treatment is in contact with the oxide semiconductor film403.

The oxygen 421 a added to the gate insulating film 402 and supplied tothe oxide semiconductor film 403 preferably has at least partly adangling bond of oxygen in the oxide semiconductor. This is because thedangling bond can be bonded with hydrogen left in the film to immobilizehydrogen (make hydrogen an immovable ion).

The oxygen doping treatment on the oxide semiconductor film may beperformed on the oxide semiconductor film before the oxide semiconductorfilm is processed into the island-shaped oxide semiconductor film orafter a source electrode layer and a drain electrode layer are stackedon the island-shaped oxide semiconductor film as long as the heattreatment is performed before that oxygen doping treatment.

Next, a conductive film for forming a source electrode layer and a drainelectrode layer (including a wiring formed of the same layer as thesource electrode layer and the drain electrode layer) is formed over thegate insulating film 402 and the oxide semiconductor film 403.

A resist mask is formed over the conductive film by a thirdphotolithography step, and is selectively etched, so that the sourceelectrode layer 405 a and the drain electrode layer 405 b are formed.Then, the resist mask is removed (see FIG. 4F).

It is preferable that etching conditions be optimized so as not to etchand cut the oxide semiconductor film 403 when the conductive film isetched. However, it is difficult to obtain etching conditions in whichonly the conductive film is etched and the oxide semiconductor film 403is not etched at all. In some cases, part of the oxide semiconductorfilm 441 is etched off through the etching of the conductive film, sothat an oxide semiconductor film having a groove portion (a depressedportion) is formed.

In this embodiment, a Ti film is used as the conductive film and anIn—Ga—Zn—O-based oxide semiconductor is used as the oxide semiconductorfilm 403, and therefore, ammonium hydrogen peroxide (a mixture ofammonia, water, and hydrogen peroxide) is used as an etchant.

The number of carriers in the highly purified oxide semiconductor film403 is significantly small (close to zero).

Next, the insulating film 407 is formed over the oxide semiconductorfilm 403, the source electrode layer 405 a, and the drain electrodelayer 405 b (see FIG. 5A).

The insulating film 407 can be formed to a thickness of at least 1 nm bya method by which an impurity such as water and hydrogen does not enterthe insulating film 407, such as a sputtering method, as appropriate.

As the insulating film 407, an inorganic insulating film such as asilicon oxide film, a silicon oxynitride film, an aluminum oxide film,an aluminum oxynitride film, or a gallium oxide film can be typicallyused.

It is far preferable that an insulating material containing acomponent/components similar to the oxide semiconductor film 403 be usedfor the insulating film 407, like the gate insulating film 402. This isbecause such a material can be fit well to the oxide semiconductor film,and therefore, this use for the insulating film 407 enables a state ofan interface between the insulating film and the oxide semiconductorfilm to be kept well. For example, in the case where the oxidesemiconductor film is formed using an In—Ga—Zn-based oxide semiconductormaterial, gallium oxide can be given as an example of the insulatingmaterial containing the component(s) similar to the oxide semiconductorfilm 403.

It is preferable to perform heat treatment after the formation of theinsulating film 407. The heat treatment is performed at a temperaturehigher than or equal to 250° C. and lower than or equal to 700° C.,preferably higher than or equal to 450° C. and lower than or equal to600° C. or less than a strain point of the substrate.

The heat treatment may be performed under an atmosphere of nitrogen,oxygen, an ultra dry air (the moisture amount is less than or equal to20 ppm (−55° C. by conversion into a dew point), preferably less than orequal to 1 ppm, far preferably less than or equal to 10 ppb, in themeasurement with the use of a dew point meter of a cavity ring downlaser spectroscopy (CRDS) system), or a rare gas (argon, helium, or thelike). The atmosphere of nitrogen, oxygen, the ultra dry air, or therare gas preferably contains water, hydrogen, or the like as less aspossible. The purity of nitrogen, oxygen, or the rare gas which isintroduced into the heat treatment apparatus is set to preferably 6N(99.9999%) or higher, far preferably 7N (99.99999%) or higher (that is,the impurity concentration is preferably 1 ppm or lower, far preferably0.1 ppm or lower).

In the case where the insulating film 407 contains oxygen and the heattreatment is performed on the state where the oxide semiconductor filmis in contact with the insulating film 407, oxygen can be furthersupplied to the oxide semiconductor film from the insulating film 407containing oxygen.

Next, oxygen doping treatment is performed on the insulating film 407.By the oxygen doping treatment on the insulating film 407, oxygen 421 cis supplied to the insulating film 407, so that oxygen is contained inthe oxide semiconductor film 403, the gate insulating film 402, and/orthe vicinity of the interface(s) of the oxide semiconductor film 403and/or the gate insulating film 402 (see FIG. 5B). In that case, theamount of oxygen contained is made to greater than the stoichiometricproportion of the insulating film 407, preferably to greater than thestoichiometric proportion and less than four times as much as thestoichiometric proportion thereof, far preferably to greater than thestoichiometric proportion and less than double of the stoichiometricproportion thereof. It can be alternatively said that the amount ofoxygen contained is made to greater than Y, where the amount of oxygencontained in a material of the insulating film in the case where thematerial is a single crystal is denoted by Y, preferably to greater thanY and less than 4Y, far preferably to greater than Y and less than 2Y.It can be further alternatively said that the amount of oxygen containedis made to greater than Z, where the amount of oxygen contained in aninsulating film which is subjected to no oxygen doping treatment isdenoted by Z, preferably to greater than Z and less than 4Z, farpreferably to greater than Z and less than 2Z. The oxygen 421 c fordoping contains an oxygen radical, an oxygen atom, and/or an oxygen ion.

For example, in the case of using an oxide insulating film thecomposition of which is represented by GaO_(x) (x>0), since thestoichiometric proportion of gallium oxide is Ga:O=1:1.5, an insulatingfilm including an oxygen-excessive region where x is greater than 1.5and less than 6 is formed. For example, in the case of using an oxideinsulating film the composition of which is represented by SiO_(x)(x>0), since the stoichiometric proportion of silicon oxide is Si:O=1:2,an insulating film including an oxygen-excessive region where x isgreater than 2 and less than 8 is formed. Such an oxygen-excessiveregion may exist in a part of the insulating film (including itsinterface). In this manner, the amount of oxygen is made to greater thanthat of hydrogen in the insulating film.

The oxygen 421 c added to (contained in) the insulating film 407preferably has at least partly a dangling bond of oxygen in the oxidesemiconductor. This is because the dangling bond can be bonded withhydrogen left in the film to immobilize hydrogen (make hydrogen animmovable ion).

Oxygen for the doping (an oxygen radical, an oxygen atom, and/or anoxygen ion) may be supplied from a radical generating apparatus with useof a gas including oxygen or from an ozone generating apparatus. Morespecifically, for example, the oxygen 421 c can be generated with anapparatus for etching treatment on a semiconductor device, an apparatusfor ashing on a mask, or the like to process the insulating film 407.

In addition, heat treatment (at 150° C. to 470° C.) may be performed onthe insulating film 407 which has been subjected to the oxygen dopingtreatment. By the heat treatment, water or hydroxide generated byreaction between the oxygen 421 c and the insulating film 407 can beremoved from the insulating film 407. The heat treatment may beperformed under an atmosphere of nitrogen, oxygen, an ultra dry air (themoisture amount is less than or equal to 20 ppm (−55° C. by conversioninto a dew point), preferably less than or equal to 1 ppm, farpreferably less than or equal to 10 ppb, in the measurement with the useof a dew point meter of a cavity ring down laser spectroscopy (CRDS)system), or a rare gas (argon, helium, or the like). The atmosphere ofnitrogen, oxygen, the ultra dry air, or the rare gas is preferablyhighly purified without containing water, hydrogen, or the like.

It is preferable to form the insulating film 409 over the insulatingfilm 407, as a protective insulating film for blocking to prevententrance of an impurity such as moisture or hydrogen into the oxidesemiconductor film 403. As the insulating film 409, it is preferable touse an inorganic insulating film such as a silicon nitride film, analuminum oxide film, or the like. For example, a silicon nitride film isformed by an RF sputtering method. An RF sputtering method is preferableas a method for forming the insulating film 409 because of its highproductivity.

Heat treatment may be performed after the insulating film is formed. Forexample, the heat treatment may be performed at a temperature higherthan or equal to 100° C. and lower than or equal to 200° C. in the airfor 1 hour to 30 hours. This heat treatment may be performed at a fixedheating temperature; alternatively, the following change in the heatingtemperature may be conducted plural times: the heating temperature isincreased from room temperature to a temperature higher than or equal to100° C. and lower than or equal to 200° C. and then decreased to roomtemperature.

Through the above process, the transistor 450 is formed (see FIG. 5C).The transistor 450 is a transistor including the oxide semiconductorfilm 403 which is highly purified and from which an impurity such ashydrogen, moisture, a hydroxyl group, or hydride (also referred to as ahydrogen compound) is removed. Therefore, variation in the electriccharacteristics of the transistor 450 is suppressed and the transistor450 is electrically stable.

In the transistor 450 including the highly-purified oxide semiconductorfilm 403 in accordance with this embodiment, the current value in anoff-state (off-current value) thereof can be low.

As described above, the oxygen doping treatment can be performed notonly on the gate insulating film 402 but also on the oxide semiconductorfilm 403 and/or the insulating film 407. The oxygen doping treatment maybe performed on either one or both of the oxide semiconductor film 403and the insulating film 407.

In addition, heat treatment (at 150° C. to 470° C.) may be performedafter the oxygen doping treatment is performed. The heat treatment maybe performed under an atmosphere of nitrogen, oxygen, an ultra dry air(the dew point is less than or equal to −60° C., preferably less than orequal to −80° C. in the measurement with the use of a dew point meter ofa cavity ring down laser spectroscopy (CRDS) system), or a rare gas(argon, helium, or the like). The atmosphere of nitrogen, oxygen, theultra dry air, or the rare gas is preferably highly purified withoutcontaining water, hydrogen, or the like.

With the heat treatment, oxygen can be supplied further effectively fromthe gate insulating film 402 subjected to the oxygen doping treatment tothe oxide semiconductor film 403.

Such a transistor including an oxide semiconductor film subjected tooxygen doping treatment is a transistor having high reliability in whichthe amount of change in threshold voltage of the transistor by thebias-temperature stress (BT) test can be reduced.

Further, in the transistor 450 including the oxide semiconductor film403, relatively high field-effect mobility can be obtained, whichenables high-speed operation. Consequently, with the above transistorprovided in a pixel portion of a semiconductor device having a displayfunction, high-quality images can be displayed. In addition, by usingthe transistor including the highly purified oxide semiconductor film, adriver circuit portion and a pixel portion can be formed over onesubstrate, whereby the number of components of the semiconductor devicecan be reduced.

In this manner, a semiconductor device including an oxide semiconductor,which has stable electric characteristics, can be provided. Accordingly,a semiconductor device with high reliability can be provided.

Embodiment 3

In Embodiment 3, another embodiment of a semiconductor device will bedescribed using FIGS. 13A to 13D. The same portions as and portionshaving functions similar to those described in Embodiment 1 or 2 can beformed in a manner similar to that described in Embodiment 1 or 2;therefore, description thereof is omitted. In addition, detaileddescription of the same portions is omitted.

In this embodiment, an example of a structure will be described in whicha source electrode layer and/or a drain electrode layer of a transistorare/is connected to a conductive layer (such as a wiring layer or apixel electrode layer). Note that this embodiment can also be applied toany of the transistors described in Embodiments 1 and 2.

As shown in FIG. 13A, a transistor 470 includes, over the substrate 400having an insulating surface, the gate electrode layer 401, the gateinsulating film 402, the oxide semiconductor film 403, the sourceelectrode layer 405 a, and the drain electrode layer 405 b.

As is described in Embodiment 2, oxygen doping is also performed on theoxide semiconductor film 403 in addition to the gate insulating film402, after the heat treatment for dehydration or dehydrogenation also ina manufacturing process of the transistor 470. The transistor 470 inthis embodiment is an example in which the source electrode layer 405 aand the drain electrode layer 405 b are formed over the oxidesemiconductor film 403 which has been subjected to heat treatment fordehydration or dehydrogenation, and then oxygen doping is performedthereon.

With this oxygen doping, an oxygen radical, an oxygen atom, or an oxygenion reaches and is delivered to (is introduced to the vicinity of topsurfaces of) the source electrode layer 405 a and the drain electrodelayer 405 b in addition to the oxide semiconductor film 403.Consequently, as shown in FIG. 13A, the top surfaces of the sourceelectrode layer 405 a and the drain electrode layer 405 b irradiatedwith the oxygen radical, the oxygen atom, or the oxygen ion may beoxidized to form metal oxide regions 404 a and 404 b between theinsulating film 407 and the source electrode layer 405 a and the drainelectrode layer 405 b. The metal oxide regions 404 a and 404 b may eachbe in the form of a film.

Next, the insulating film 407 and the insulating film 409 aresequentially stacked over the transistor 470 (see FIG. 13B).

In the case of FIG. 13B, openings 455 a and 455 b where, over theinsulating film 409, conductive layers connected to the source electrodelayer 405 a and the drain electrode layer 405 b are formed arepreferably formed so that parts of the metal oxide regions 404 a and 404b having high resistance are removed to expose parts of the sourceelectrode layer 405 a and the drain electrode layer 405 b having lowresistance (see FIG. 13C). Parts of the insulating film 409, theinsulating film 407, and the metal oxide regions 404 a and 404 b areremoved to form the openings 455 a and 455 b. The source electrode layer405 a and the drain electrode layer 405 b are partly removed to havedepressions. The oxygen concentrations of regions of the sourceelectrode layer 405 a and the drain electrode layer 405 b, which areexposed on the bottom surfaces of the depressions, are lower than thoseof regions of the metal oxide regions 404 a and 404 b, which are in thetop surfaces of the source electrode layer 405 a and the drain electrodelayer 405 b.

For example, the parts of the source electrode layer 405 a and the drainelectrode layer 405 b may be removed from the top surfaces by athickness one half or less than (preferably one third or less than) thethickness of the source electrode layer 405 a, the drain electrode layer405 b in the openings 455 a and 455 b to remove parts of the metal oxideregions 404 a and 404 b formed in the surfaces of the source electrodelayer 405 a and the drain electrode layer 405 b.

Next, conductive layers 456 a and 456 b are formed in contact with thesource electrode layer 405 a and the drain electrode layer 405 b exposedin the openings 455 a and 455 b (see FIG. 13D). The conductive layers456 a and 456 b are formed directly in contact with the source electrodelayer 405 a and the drain electrode layer 405 b having low resistancewithout the metal oxide regions 404 a and 404 b having high resistanceprovided therebetween; thus, favorable electrical connection (contact)can be made.

An insulating film may be formed over the conductive layers 456 a and456 b, as a protective layer to cover the transistor 470. Moreover, bycovering the insulating film, it is possible to prevent impurities suchas hydrogen and moisture from entering the oxide semiconductor film 403from the openings 455 a and 455 b.

In this manner, favorable electrical connection of a transistor can beobtained and a semiconductor device including an oxide semiconductorwith stable electrical characteristics can be provided. Therefore, asemiconductor device with high reliability can be provided.

Embodiment 4

In Embodiment 4, an example of a plasma apparatus (also referred to asan ashing apparatus) which can be used for oxygen doping treatment willbe described. This apparatus is industrially suitable as compared to anion implantation apparatus or the like because the apparatus can beapplicable for a large-sized substrate of the fifth generation or later,for example.

FIG. 14A illustrates an example of a top view of a single wafermulti-chamber equipment. FIG. 14B illustrates an example of across-sectional view of a plasma apparatus (also referred to as anashing apparatus) used for oxygen doping.

The single wafer multi-chamber equipment illustrated in FIG. 14Aincludes three plasma apparatuses 10 each of which is shown in FIG. 14B,a substrate supply chamber 11 including three cassette ports 14 forholding a process substrate, a load lock chamber 12, a transfer chamber13, and the like. A substrate supplied to the substrate supply chamber11 is transferred through the load lock chamber 12 and the transferchamber 13 to a vacuum chamber 15 in the plasma apparatus 10 and issubjected to oxygen doping. The substrate which has been subjected tooxygen doping is transferred from the plasma apparatus 10, through thetransfer chamber 13 and the load lock chamber 12 to the substrate supplychamber. A transfer robot for transferring a process substrate isprovided in each of the substrate supply chamber 11 and the transferchamber 13.

Referring to FIG. 14B, the plasma apparatus 10 includes the vacuumchamber 15. A plurality of gas outlets and an ICP coil (an inductivelycoupled plasma coil) 16 which is a generation source of plasma areprovided on a top portion of the vacuum chamber 15.

The number of gas outlets arranged in a center portion, seen from thetop of the plasma apparatus 10 is 12. Each of the gas outlets isconnected to a gas supply source for supplying an oxygen gas, via a gasflow path 17. The gas supply source includes a mass flow controller andthe like and can supply an oxygen gas to the gas flow pass 17 at adesired flow rate (which is greater than 0 sccm and less than or equalto 1000 sccm). The oxygen gas supplied from the gas supply source issupplied from the gate flow pass 17, through the 12 gas outlets, intothe vacuum chamber 15.

The ICP coil 16 includes a plurality of strip-like conductors each ofwhich has a spiral form. One end of each of the conductors iselectrically connected to a first high-frequency power source 18 (13.56MHz) via a matching circuit for controlling impedance, and the other endthereof is grounded.

A substrate stage 19 functioning as a bottom electrode is provided in alower portion of the vacuum chamber. By an electrostatic chuck or thelike provided for the substrate stage 19, a process substrate 20 is heldon the substrate stage so as to be detachable. The substrate stage 19 isprovided with a heater as a heating system and a He gas flow pass as acooling system. The substrate stage is connected to a secondhigh-frequency power source 21 (3.2 MHz) for applying a substrate biaspower.

In addition, the vacuum chamber 15 is provided with an exhaust port andan automatic pressure control valve (also referred to as an APC) 22. TheAPC is connected to a turbo molecular pump 23 and connected to a drypump 24 via the turbo molecular pump 23. The APC controls the insidepressure of the vacuum chamber. The turbo molecular pump 23 and the drypump 24 reduce the inside pressure of the vacuum chamber 15.

Next, described is an example in which plasma is generated in the vacuumchamber 15 illustrated in FIG. 14B, and oxygen doping is performed on anoxide semiconductor film or a gate insulating film provided for theprocess substrate 20.

First, the inside pressure of the vacuum chamber 15 is held at a desiredpressure by operating the turbo molecular pump 23, the dry pump 24, andthe like, and then, the process substrate 20 is installed on thesubstrate stage in the vacuum chamber 15. The process substrate 20 heldon the substrate stage has at least an oxide semiconductor film or agate insulating film. In this embodiment, the inside pressure of thevacuum chamber 15 is held at 1.33 Pa. The flow rate of the oxygen gassupplied through the gas outlets into the vacuum chamber 15 is set at250 sccm.

Next, a high-frequency power is applied from the first high-frequencypower source 18 to the ICP coil 16, thereby generating plasma. Then, astate in which plasma is being generated is kept for a certain period(longer than or equal to 30 seconds and shorter than or equal to 600seconds). The high-frequency power applied to the ICP coil 16 is greaterthan or equal to 1 kW and less than or equal to 10 kW. In thisembodiment, the high-frequency power is set at 6000 W. At that time, asubstrate bias power may be applied from the second high-frequency powersource 21 to the substrate stage. In this embodiment, the power used forapplying the substrate bias power is set at 1000 W.

In this embodiment, the state in which plasma is being generated is keptfor 60 seconds and then, the process substrate 20 is transferred fromthe vacuum chamber 15. In this manner, oxygen doping can be performed onthe oxide semiconductor film or the gate insulating film provided forthe process substrate 20.

Embodiment 5 can be implemented in appropriate combination with any ofthe structures described in the other embodiments.

Embodiment 5

A semiconductor device having a display function (also referred to as adisplay device) can be manufactured using the transistor exemplified inany of Embodiments 1 to 3. Moreover, part or all of driver circuitrywhich include the transistor can be formed over a substrate where apixel portion is formed, whereby a system-on-panel can be obtained.

In FIG. 12A, a sealant 4005 is provided so as to surround a pixelportion 4002 provided over a first substrate 4001, and the pixel portion4002 is sealed with a second substrate 4006. In FIG. 12A, a signal linedriver circuit 4003 and a scan line driver circuit 4004 which are formedusing a single crystal semiconductor film or a polycrystallinesemiconductor film over another substrate are mounted in a region thatis different from the region surrounded by the sealant 4005 over thefirst substrate 4001. Various signals and potentials are supplied to thesignal line driver circuit 4003, the scan line driver circuit 4004, andthe pixel portion 4002 from flexible printed circuits (FPCs) 4018 a and4018 b.

In FIGS. 12B and 12C, the sealant 4005 is provided so as to surround thepixel portion 4002 and the scan line driver circuit 4004 which areprovided over the first substrate 4001. The second substrate 4006 isprovided over the pixel portion 4002 and the scan line driver circuit4004. Accordingly, the pixel portion 4002 and the scan line drivercircuit 4004 are sealed together with the display element, by the firstsubstrate 4001, the sealant 4005, and the second substrate 4006. InFIGS. 12B and 12C, the signal line driver circuit 4003 which is formedusing a single crystal semiconductor film or a polycrystallinesemiconductor film over another substrate is mounted in a region that isdifferent from the region surrounded by the sealant 4005 over the firstsubstrate 4001. In FIGS. 12B and 12C, various signals and potential aresupplied to the signal line driver circuit 4003, the scan line drivercircuit 4004, and the pixel portion 4002 from an FPC 4018.

Although FIGS. 12B and 12C each illustrate an example in which thesignal line driver circuit 4003 is separately formed and mounted on thefirst substrate 4001, an embodiment of the present invention is notlimited to this structure. The scan line driver circuit may beseparately formed and then mounted, or only part of the signal linedriver circuit or part of the scan line driver circuit may be separatelyformed and then mounted.

A connection method of a separately formed driver circuit is notparticularly limited; a chip on glass (COG) method, a wire bondingmethod, a tape automated bonding (TAB) method, or the like can be used.FIG. 12A illustrates an example in which the signal line driver circuit4003 and the scan line driver circuit 4004 are mounted by a COG method.FIG. 12B illustrates an example in which the signal line driver circuit4003 is mounted by a COG method. FIG. 12C illustrates an example inwhich the signal line driver circuit 4003 is mounted by a TAB method.

In addition, the display device includes a panel in which the displayelement is sealed, and a module in which an IC or the like including acontroller is mounted on the panel.

Note that the display device in this specification means an imagedisplay device, a display device, or a light source (including alighting device). Furthermore, the display device also includes thefollowing in its category: a module to which a connector such as an FPC,a TAB tape, or a TCP is attached; a module having a TAB tape or a TCP atthe tip of which a printed wiring board is provided; and a module inwhich an integrated circuit (IC) is directly mounted on a displayelement by a COG method.

The pixel portion and the scan line driver circuit provided over thefirst substrate include a plurality of transistors; any of thetransistors which are described in Embodiments 1 to 3 can be appliedthereto.

As the display element provided in the display device, a liquid crystalelement (also referred to as a liquid crystal display element) or alight-emitting element (also referred to as a light-emitting displayelement) can be used. The light-emitting element includes, in itscategory, an element whose luminance is controlled by a current or avoltage, and specifically includes, in its category, an inorganicelectroluminescent (EL) element, an organic EL element, and the like.Furthermore, a display medium whose contrast is changed by an electriceffect, such as electronic ink, can be used.

An embodiment of the semiconductor device is described with reference toFIGS. 6 to 8. FIGS. 6 to 8 correspond to cross-sectional views alongline M-N in FIG. 12B.

As illustrated in FIGS. 6 to 8, the semiconductor device includes aconnection terminal electrode 4015 and a terminal electrode 4016. Theconnection terminal electrode 4015 and the terminal electrode 4016 areelectrically connected to a terminal included in the FPC 4018 via ananisotropic conductive film 4019.

The connection terminal electrode 4015 is formed using the sameconductive film as a first electrode layer 4030, and the terminalelectrode 4016 is formed using the same conductive film as source anddrain electrodes of a transistor 4010 and a transistor 4011.

The pixel portion 4002 and the scan line driver circuit 4004 which areprovided over the first substrate 4001 include a plurality oftransistors. In FIGS. 6 to 8, the transistor 4010 included in the pixelportion 4002 and the transistor 4011 included in the scan line drivercircuit 4004 are illustrated as an example. In FIG. 6, insulating films4020 and 4024 are provided over the transistor 4010 and the transistor4011. In FIGS. 7 and 8, an insulating layer 4021 is further provided. Aninsulating film 4023 is an insulating film functioning as a base film.

In this embodiment, any transistors described in Embodiments 1 to 3 canbe applied to the transistor 4010 and the transistor 4011. Variation inelectric characteristics of the transistor 4010 and the transistor 4011is suppressed and the transistor 4010 and the transistor 4011 areelectrically stable. Accordingly, highly reliable semiconductor devicescan be provided as the semiconductor devices illustrated in FIGS. 6 to8.

In addition, in this embodiment, a conductive layer is provided over theinsulating layer so as to overlap with a channel formation region of anoxide semiconductor film in the transistor 4011 for the driver circuit.By providing the conductive layer so as to overlap with the channelformation region of the oxide semiconductor film, the amount of changein the threshold voltage of the transistor 4011 by the BT test can befurther reduced. The potential of the conductive layer may be the sameas or different from that of a gate electrode of the transistor 4011,and the conductive layer can be functioned as a second gate electrode.The potential of the conductive layer may be GND, 0V, or in a floatingstate.

The conductive layer also functions to block an external electric field,that is, to prevent an external electric field (particularly, to preventstatic electricity) from effecting the inside (a circuit portionincluding a transistor). The blocking function of the conductive layerenables the variation in electrical characteristics of the transistordue to the effect of external electric field such as static electricityto be prevented.

The transistor 4010 provided in the pixel portion 4002 is electricallyconnected to the display element in a display panel. A variety ofdisplay elements can be used as the display element as long as displaycan be performed.

An example of a liquid crystal display device using a liquid crystalelement as the display element is described in FIG. 6. In FIG. 6, aliquid crystal element 4013 which is a display element includes thefirst electrode layer 4030, the second electrode layer 4031, and aliquid crystal layer 4008. An insulating film 4032 and an insulatingfilm 4033 which serve as alignment films are provided so that the liquidcrystal layer 4008 is provided therebetween. The second electrode layer4031 is provided on the second substrate 4006 side, and the firstelectrode layer 4030 and the second electrode layer 4031 are stackedwith the liquid crystal layer 4008 provided therebetween.

A spacer 4035 is a columnar spacer obtained by selective etching of aninsulating film and is provided in order to control the thickness (acell gap) of the liquid crystal layer 4008. Note the spacer is notlimited to a columnar spacer, and, for example, a spherical spacer maybe used.

In the case where a liquid crystal element is used as the displayelement, a thermotropic liquid crystal, a low-molecular liquid crystal,a high-molecular liquid crystal, a polymer dispersed liquid crystal, aferroelectric liquid crystal, an anti-ferroelectric liquid crystal, orthe like can be used. Such a liquid crystal material exhibits acholesteric phase, a smectic phase, a cubic phase, a chiral nematicphase, an isotropic phase, or the like depending on a condition.

Alternatively, a liquid crystal exhibiting a blue phase for which analignment film is unnecessary may be used. A blue phase is one of liquidcrystal phases, which is generated just before a cholesteric phasechanges into an isotropic phase while temperature of cholesteric liquidcrystal is increased. Since the blue phase appears only in a narrowtemperature range, a liquid crystal composition in which 5 weightpercent or more of a chiral material is mixed is used for the liquidcrystal layer in order to improve the temperature range. The liquidcrystal composition which includes a liquid crystal exhibiting a bluephase and a chiral agent has a short response time of 1 msec or less,has optical isotropy, which makes the alignment process unneeded, andhas a small viewing angle dependence. In addition, since an alignmentfilm does not need to be provided and rubbing treatment is unnecessary,electrostatic discharge damage caused by the rubbing treatment can beprevented and defects and damage of the liquid crystal display devicecan be reduced in the manufacturing process. Thus, productivity of theliquid crystal display device can be increased.

The specific resistivity of the liquid crystal material is 1×10⁹ Ω·cm ormore, preferably 1×10¹¹ Ω·cm or more, far preferably 1×10¹² Ω·cm ormore. The value of the specific resistivity in this specification ismeasured at 20° C.

The size of a storage capacitor formed in the liquid crystal displaydevice is set considering the leakage current of the transistor providedin the pixel portion or the like so that electrical charge can be heldfor a predetermined period. By using the transistor including the highlypurified oxide semiconductor film, it is enough to provide a storagecapacitor having a capacitance that is ⅓ or less, preferably ⅕ or lessof a liquid crystal capacitance of each pixel.

In the transistor used in this embodiment, which includes the highlypurified oxide semiconductor film, the current in an off state (theoff-state current) can be made small. Accordingly, an electrical signalsuch as an image signal can be held for a long period, and a writinginterval can be set long in a state where power is being supplied.Accordingly, the frequency of refresh operation can be reduced, whichleads to an effect of suppressing power consumption.

In addition, the transistor including the highly purified oxidesemiconductor film used in this embodiment can have relatively highfield-effect mobility and thus is capable of high speed operation.Therefore, by using the transistor in the pixel portion of the liquidcrystal display device, a high-quality image can be displayed. Inaddition, since the transistors can be separately provided in a drivercircuit portion and a pixel portion over one substrate, the number ofcomponents of the liquid crystal display device can be reduced.

For the liquid crystal display device, a twisted nematic (TN) mode, anin-plane-switching (IPS) mode, a fringe field switching (FFS) mode, anaxially symmetric aligned micro-cell (ASM) mode, an optical compensatedbirefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, anantiferroelectric liquid crystal (AFLC) mode, or the like can be used.

A normally black liquid crystal display device such as a transmissiveliquid crystal display device employing a vertical alignment (VA) modemay be used. The vertical alignment mode is a method of controllingalignment of liquid crystal molecules of a liquid crystal display panel,in which liquid crystal molecules are aligned vertically to a panelsurface when no voltage is applied. Some examples are given as thevertical alignment mode. For example, a multi-domain vertical alignment(MVA) mode, a patterned vertical alignment (PVA) mode, an advanced superview (ASV) mode, or the like can be used. Moreover, it is possible touse a method called domain multiplication or multi-domain design, inwhich a pixel is divided into some regions (subpixels) and molecules arealigned in different directions in their respective regions.

In the display device, a black matrix (a light-blocking layer), anoptical member (an optical substrate) such as a polarizing member, aretardation member, or an anti-reflection member, and the like areprovided as appropriate. For example, circular polarization may beprovided with a polarizing substrate and a retardation substrate. Inaddition, a backlight, a side light, or the like may be used as thelight source.

In addition, it is possible to employ a time-division display method (afield-sequential driving method) with the use of a plurality oflight-emitting diodes (LEDs) as a backlight. A field-sequential drivingmethod enables color display without using a color filter.

As a display method in the pixel portion, a progressive method, aninterlace method or the like can be employed. Further, color elementscontrolled in a pixel for color display are not limited to three colorsof R, G, and B (R, G, and B correspond to red, green, and blue,respectively). For example, the following can be used: R, G, B, and W (Wcorresponds to white); or R, G, B, and one or more of yellow, cyan,magenta, and the like. The sizes of display regions may be differentbetween respective dots of the color elements. The present invention isnot limited to the application to a display device for color display butcan also be applied to a display device for monochrome display.

Alternatively, as the display element included in the display device, alight-emitting element utilizing electroluminescence can be used.Light-emitting elements utilizing electroluminescence are classifieddepending on whether a light-emitting material is an organic compound oran inorganic compound. In general, the former is referred to as anorganic EL element, and the latter is referred to as an inorganic ELelement.

In an organic EL element, by application of voltage to a light-emittingelement, electrons and holes are injected from a pair of electrodes intoa layer containing a light-emitting organic compound, and current flows.The carriers (electrons and holes) are recombined, and thus, thelight-emitting organic compound is excited. The light-emitting organiccompound returns to a ground state from the excited state, therebyemitting light. Owing to such a mechanism, this light-emitting elementis referred to as a current-excitation light-emitting element.

The inorganic EL elements are classified depending on the elementstructure into a dispersion-type inorganic EL element and a thin-filminorganic EL element. A dispersion-type inorganic EL element has alight-emitting layer where particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination type light emission that utilizes a donorlevel and an acceptor level. A thin-film inorganic EL element has astructure where a light-emitting layer is sandwiched between dielectriclayers, which are further sandwiched between electrodes, and its lightemission mechanism is localized type light emission that utilizesinner-shell electron transition of metal ions. An example using anorganic EL element as a light-emitting element is described here.

In order to extract light emitted from the light-emitting element, atleast one of a pair of electrodes is transparent. The transistor and thelight-emitting element are provided over the substrate. Thelight-emitting element have the following emission structure: a topemission structure in which light emission is extracted through thesurface opposite to the substrate; a bottom emission structure in whichlight emission is extracted through the surface on the substrate side;or a dual emission structure in which light emission is extractedthrough the surface opposite to the substrate and the surface on thesubstrate side.

An example of a light-emitting device in which a light-emitting elementis used as the display element is illustrated in FIG. 7. Alight-emitting element 4513 which is a display element is electricallyconnected to the transistor 4010 provided in the pixel portion 4002. Astructure of the light-emitting element 4513 is not limited to thestacked-layer structure including the first electrode layer 4030, anelectroluminescent layer 4511, and the second electrode layer 4031,which is shown in FIG. 7. The structure of the light-emitting element4513 can be changed as appropriate depending on a direction in whichlight is extracted from the light-emitting element 4513, or the like.

A partition wall 4510 is formed using an organic insulating material oran inorganic insulating material. It is particularly preferable that thepartition wall 4510 be formed using a photosensitive resin material tohave an opening over the first electrode layer 4030 so that the sidewallof the opening has a tilted surface with continuous curvature.

The electroluminescent layer 4511 may be formed using a single layer ora plurality of layers stacked.

A protective film may be formed over the second electrode layer 4031 andthe partition wall 4510 in order to prevent entry of oxygen, hydrogen,moisture, carbon dioxide, or the like into the light-emitting element4513. As the protective film, a silicon nitride film, a silicon nitrideoxide film, a diamond like carbon (DLC) film, or the like can be formed.In addition, in a space which is formed with the first substrate 4001,the second substrate 4006, and the sealant 4005, a filler 4514 isprovided for sealing. It is preferable that a panel be packaged (sealed)with a protective film (such as a laminate film or an ultravioletcurable resin film) or a cover material with high air-tightness andlittle degasification so that the panel is not exposed to the outsideair, in this manner.

As the filler 4514, an ultraviolet curable resin or a thermosettingresin can be used as well as an inert gas such as nitrogen or argon. Forexample, PVC (polyvinyl chloride), acrylic, polyimide, an epoxy resin, asilicone resin, PVB (polyvinyl butyral), or EVA (ethylene vinyl acetate)can be used. For example, nitrogen is used for the filler.

In addition, if needed, an optical film, such as a polarizing plate, acircularly polarizing plate (including an elliptically polarizingplate), a retardation plate (a quarter-wave plate or a half-wave plate),or a color filter, may be provided as appropriate for a light-emittingsurface of the light-emitting element. Further, the polarizing plate orthe circularly polarizing plate may be provided with an anti-reflectionfilm. For example, anti-glare treatment by which reflected light can bediffused by surface roughness so as to reduce the glare can beperformed.

Further, an electronic paper in which electronic ink is driven can beprovided as the display device. The electronic paper is also called anelectrophoretic display device (electrophoretic display) and hasadvantages in that it exhibits the same level of readability as regularpaper, it exhibits less power consumption than other display devices,and it can be in a thin and light form.

An electrophoretic display device can have various modes. Anelectrophoretic display device contains a plurality of microcapsulesdispersed in a solvent or a solute, each microcapsule containing firstparticles which are positively charged and second particles which arenegatively charged. By applying an electric field to the microcapsules,the particles in the microcapsules move in opposite directions to eachother and only the color of the particles gathering on one side isdisplayed. Note that the first particles and the second particles eachcontain pigment and do not move without an electric field. Moreover, thefirst particles and the second particles have different colors (one ofwhich may be colorless).

Thus, an electrophoretic display device is a display device thatutilizes a so-called dielectrophoretic effect by which a substancehaving a high dielectric constant moves to a high-electric field region.

A solution in which the above microcapsules are dispersed in a solventis referred to as electronic ink. This electronic ink can be printed ona surface of glass, plastic, cloth, paper, or the like. Furthermore, byusing a color filter or particles containing a pigment, color displaycan also be achieved.

The first particles and the second particles in the microcapsules may beformed of one kind of material selected from a conductive material, aninsulating material, a semiconductor material, a magnetic material, aliquid crystal material, a ferroelectric material, an electroluminescentmaterial, an electrochromic material, and a magnetophoretic material, ora composite material of any of these.

As the electronic paper, a display device using a twisting ball displaysystem can be used. The twisting ball display system refers to a methodin which spherical particles each colored in black and white arearranged between a first electrode layer and a second electrode layerwhich are electrode layers used for a display element, and a potentialdifference is generated between the first electrode layer and the secondelectrode layer to control orientation of the spherical particles, sothat display is performed.

FIG. 8 illustrates active matrix electronic paper as an embodiment of asemiconductor device. The electronic paper shown in FIG. 8 is an exampleof a display device using the twisting ball display system.

Between the first electrode layer 4030 connected to the transistor 4010and the second electrode layer 4031 provided for the second substrate4006, spherical particles 4613 each of which includes a black region4615 a, a white region 4615 b, and a cavity 4612 which is filled withliquid around the black region 4615 a and the white region 4615 b, areprovided. A space around the spherical particles 4613 is filled with afiller 4614 such as a resin. The second electrode layer 4031 correspondsto a common electrode (counter electrode). The second electrode layer4031 is electrically connected to a common potential line.

In FIGS. 6 to 8, a flexible substrate as well as a glass substrate canbe used as any of the first substrate 4001 and the second substrate4006. For example, a plastic substrate having light-transmittingproperties can be used. As plastic, a fiberglass-reinforced plastics(FRP) plate, a polyvinyl fluoride (PVF) film, a polyester film, or anacrylic resin film can be used. In addition, a sheet with a structure inwhich an aluminum foil is sandwiched between PVF films or polyesterfilms can be used.

The insulating film 4020 can be formed using an inorganic insulatingmaterial such as silicon oxide, silicon oxynitride, hafnium oxide,aluminum oxide, or gallium oxide. A manufacturing method of theinsulating film 4020 is not particularly limited; for example, a filmformation method such as a plasma CVD method or a sputtering method canbe used. The sputtering method is preferable in that hydrogen, water,and the like are unlikely to enter a film to be formed.

The insulating film 4024 can be formed with a single-layer structure ora multi-layer structure using one or more of a silicon nitride film, asilicon nitride oxide film, an aluminum oxide film, an aluminum nitridefilm, an aluminum oxynitride film, and an aluminum nitride oxide film bya sputtering method. The insulating film 4024 functions as a protectivefilm of the transistor(s).

The insulating layer 4021 can be formed using an inorganic insulatingmaterial or an organic insulating material. The insulating layer 4021may be formed using a heat-resistant organic insulating material such asan acrylic resin, polyimide, a benzocyclobutene-based resin, polyamide,or an epoxy resin, which is preferable as a planarizing insulating film.As well as such an organic insulating material, it is possible to use alow-dielectric constant material (a low-k material), a siloxane basedresin, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), orthe like. The insulating layer may be formed by stacking a plurality ofinsulating films formed of these materials.

There is no particular limitation on the method for forming theinsulating layer 4021; the insulating layer 4021 can be formed,depending on the material, by a sputtering method, a spin coatingmethod, a dipping method, spray coating, a droplet discharge method(e.g., an inkjet method, screen printing, or offset printing), rollcoating, curtain coating, knife coating, or the like.

The display device displays an image by transmitting light from thelight source or the display element. Therefore, the substrate and thethin films such as the insulating film and the conductive film providedfor the pixel portion where light is transmitted have light-transmittingproperties with respect to light in the visible-light wavelength range.

The first electrode layer and the second electrode layer (each of whichmay be called a pixel electrode layer, a common electrode layer, acounter electrode layer, or the like) for applying voltage to thedisplay element may have light-transmitting properties orlight-reflecting properties, which depends on the direction in whichlight is extracted, the position where the electrode layer is provided,the pattern structure of the electrode layer, and the like.

Any of the first electrode layer 4030 and the second electrode layer4031 can be formed using a light-transmitting conductive material suchas indium oxide containing tungsten oxide, indium zinc oxide containingtungsten oxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium tin oxide (hereinafter referred to asITO), indium zinc oxide, or indium tin oxide to which silicon oxide isadded.

Any of the first electrode layer 4030 and the second electrode layer4031 can be formed using one or more kinds of materials selected frommetals such as tungsten (W), molybdenum (Mo), zirconium (Zr), hafnium(Hf), vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr), cobalt(Co), nickel (Ni), titanium (Ti), platinum (Pt), aluminum (Al), copper(Cu), and silver (Ag); alloys of these metals; and nitrides of thesemetals.

Since the transistor is easily broken owing to static electricity or thelike, a protective circuit for protecting the driver circuit ispreferably provided. The protective circuit is preferably formed using anon-linear element.

In this manner, by using any of the transistors described in Embodiments1 to 3, a highly reliable semiconductor device can be provided.

Embodiment 5 can be implemented in appropriate combination with any ofthe structures described in the other embodiments.

Embodiment 6

A semiconductor device having an image sensor function for reading dataof an object can be manufactured with the use of any transistorexemplified in Embodiments 1 to 3.

An example of a semiconductor device having an image sensor function isillustrated in FIG. 9A. FIG. 9A illustrates an equivalent circuit of aphoto sensor, and FIG. 9B is a cross-sectional view illustrating part ofthe photo sensor.

One electrode of a photodiode 602 is electrically connected to aphotodiode reset signal line 658, and the other electrode thereof iselectrically connected to a gate of a transistor 640. One of a sourceand a drain of the transistor 640 is electrically connected to a photosensor reference signal line 672, and the other of the source and thedrain thereof is electrically connected to one of a source and a drainof a transistor 656. A gate of the transistor 656 is electricallyconnected to a gate signal line 659, and the other of the source and thedrain thereof is electrically connected to a photo sensor output signalline 671.

Note that in circuit diagrams in this specification, a transistorincluding an oxide semiconductor film is denoted with a symbol “OS” sothat it can be identified as a transistor including an oxidesemiconductor film. The transistor 640 and the transistor 656 in FIG. 9Aare transistors each including an oxide semiconductor film.

FIG. 9B is a cross-sectional view of the photodiode 602 and thetransistor 640 in the photo sensor. The photodiode 602 functioning as asensor and the transistor 640 are provided over a substrate 601 (a TFTsubstrate) having an insulating surface. A substrate 613 is providedover the photodiode 602 and the transistor 640 with the use of anadhesion layer 608.

An insulating film 631, a protective insulating film 632, a firstinterlayer insulating layer 633, and a second interlayer insulatinglayer 634 are provided over the transistor 640. The photodiode 602 isprovided over the first interlayer insulating layer 633. In thephotodiode 602, a first semiconductor layer 606 a, a secondsemiconductor layer 606 b, and a third semiconductor layer 606 c arestacked in this order over the first interlayer insulating layer 633between an electrode layer 641 provided over the first interlayerinsulating layer 633 and an electrode layer 642 provided over the secondinterlayer insulating layer 634.

In this embodiment, any of the transistors described in Embodiments 1 to3 can be applied to the transistor 640. In the transistor 640 and thetransistor 656, variation in electrical characteristics is suppressed,and the transistor 640 and the transistor 656 are electrically stable.Accordingly, a highly reliable semiconductor device can be provided asthe semiconductor device of this embodiment described in FIGS. 9A and9B.

The electrode layer 641 is electrically connected to a conductive layer643 formed over the second interlayer insulating layer 634, and theelectrode layer 642 is electrically connected to a gate electrode 645through a electrode layer 644. The gate electrode 645 is electricallyconnected to the gate electrode of the transistor 640, and thephotodiode 602 is electrically connected to the transistor 640.

Here, a pin photodiode in which a semiconductor layer having a p-typeconductivity as the first semiconductor layer 606 a, a high-resistancesemiconductor layer (i-type semiconductor layer) as the secondsemiconductor layer 606 b, and a semiconductor layer having an n-typeconductivity as the third semiconductor layer 606 c are stacked isillustrated as an example.

The first semiconductor layer 606 a is a p-type semiconductor layer andcan be formed using an amorphous silicon film containing an impurityelement imparting the p-type conductivity. The first semiconductor layer606 a is formed by a plasma CVD method with use of a semiconductorsource gas containing an impurity element belonging to Group 13 (such asboron (B)). As the semiconductor source gas, silane (SiH₄) may be used.Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or the like may beused. Further alternatively, an amorphous silicon film which does notcontain an impurity element may be formed, and then, an impurity elementmay be introduced to the amorphous silicon film by a diffusion method oran ion implantation method. Heating or the like may be conducted afterintroducing the impurity element by an ion implantation method or thelike in order to diffuse the impurity element. In that case, as a methodof forming the amorphous silicon film, an LPCVD method, a chemical vapordeposition method, a sputtering method, or the like may be used. Thefirst semiconductor layer 606 a is preferably formed to have a thicknessgreater than or equal to 10 nm and less than or equal to 50 nm.

The second semiconductor layer 606 b is an i-type semiconductor layer(intrinsic semiconductor layer) and is formed using an amorphous siliconfilm. As for formation of the second semiconductor layer 606 b, anamorphous silicon film is formed with use of a semiconductor source gasby a plasma CVD method. As the semiconductor source gas, silane (SiH₄)may be used. Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or thelike may be used. The second semiconductor layer 606 b may be formed byan LPCVD method, a chemical vapor deposition method, a sputteringmethod, or the like. The second semiconductor layer 606 b is preferablyformed to have a thickness greater than or equal to 200 nm and less thanor equal to 1000 nm.

The third semiconductor layer 606 c is an n-type semiconductor layer andis formed using an amorphous silicon film containing an impurity elementimparting the n-type conductivity. The third semiconductor layer 606 cis formed by a plasma CVD method with use of a semiconductor source gascontaining an impurity element belonging to Group 15 (e.g., phosphorus(P)). As the semiconductor source gas, silane (SiH₄) may be used.Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or the like may beused. Further alternatively, an amorphous silicon film which does notcontain an impurity element may be formed, and then, an impurity elementmay be introduced to the amorphous silicon film by a diffusion method oran ion implantation method. Heating or the like may be conducted afterintroducing the impurity element by an ion injecting method or the likein order to diffuse the impurity element. In that case, as a method offorming the amorphous silicon film, an LPCVD method, a chemical vapordeposition method, a sputtering method, or the like may be used. Thethird semiconductor layer 606 c is preferably formed to have a thicknessgreater than or equal to 20 nm and less than or equal to 200 nm.

Any of the first semiconductor layer 606 a, the second semiconductorlayer 606 b, and the third semiconductor layer 606 c is not necessarilyformed using an amorphous semiconductor, and may be formed using apolycrystalline semiconductor, or a micro crystalline semiconductor (asemi-amorphous semiconductor: SAS).

The microcrystalline semiconductor belongs to a metastable state of anintermediate between amorphous and single crystalline, considering Gibbsfree energy. That is, the microcrystalline semiconductor is asemiconductor having a third state which is stable in terms of freeenergy and has a short range order and lattice distortion. Columnar-likeor needle-like crystals grow in a normal direction with respect to asubstrate surface. The Raman spectrum of microcrystalline silicon, thatis a typical example of a microcrystalline semiconductor, is located inlower wave numbers than 520 cm⁻¹, which represents a peak of the Ramanspectrum of single crystal silicon. That is, the peak of the Ramanspectrum of the microcrystalline silicon exists between 520 cm⁻¹ whichrepresents single crystal silicon and 480 cm⁻¹ which representsamorphous silicon. The semiconductor contains hydrogen or halogen of atleast 1 at. % to terminate a dangling bond. Moreover, microcrystallinesilicon is made to contain a rare gas element such as helium, neon,argon, or krypton to further enhance lattice distortion, wherebystability is increased and a favorable microcrystalline semiconductorfilm can be obtained.

The microcrystalline semiconductor film can be formed by ahigh-frequency plasma CVD method with a frequency of several tens ofmegahertz to several hundreds of megahertz or using a microwave plasmaCVD apparatus with a frequency of 1 GHz or more. Typically, themicrocrystalline semiconductor film can be formed by using a gasobtained by diluting SiH₄, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, or SiF₄, withhydrogen. Further, with a dilution with one or plural kinds of rare gaselements selected from helium, neon, argon, and krypton in addition tosilicon hydride and hydrogen, the microcrystalline semiconductor filmcan be formed. In that case, the flow ratio of hydrogen to siliconhydride is 5:1 to 200:1, preferably 50:1 to 150:1, far preferably 100:1.Further, a hydrocarbon gas such as CH₄ or C₂H₆, a gas containinggermanium such as GeH₄ or GeF₄, F₂, or the like may be mixed into thegas containing silicon.

In addition, since the mobility of holes generated by a photoelectriceffect is lower than that of electrons, a pin photodiode exhibits bettercharacteristics when a surface on the p-type semiconductor layer side isused as a light-receiving plane. Here, an example in which light 622received by the photodiode 602 from a surface of the substrate 601, overwhich the pin photodiode is formed, is converted into electric signalsis described. Further, light from the semiconductor layer having aconductivity type opposite from that of the semiconductor layer on thelight-receiving plane is disturbance light; therefore, the electrodelayer 642 on the semiconductor layer having the opposite conductivitytype is preferably formed from a light-blocking conductive film. Notethat a surface on the n-type semiconductor layer side can alternativelybe used as the light-receiving plane.

For reduction of the surface roughness, an insulating layer functioningas a planarizing insulating film is preferably used as any of the firstinterlayer insulating layer 633 and the second interlayer insulatinglayer 634. Any of the first interlayer insulating layer 633 and thesecond interlayer insulating layer 634 can be formed using, for example,an organic insulating material such as polyimide, an acrylic resin, abenzocyclobutene-based resin, polyamide, or an epoxy resin. As well assuch an organic insulating material, it is possible to use a singlelayer or multi layers of a low-dielectric constant material (a low-kmaterial), a siloxane-based resin, phosphosilicate glass (PSG),borophosphosilicate glass (BPSG), or the like.

Any of the insulating film 631, the protective insulating film 632, thefirst interlayer insulating layer 633, and the second interlayerinsulating layer 634 can be formed using an insulating material by asputtering method, a spin coating method, a dipping method, spraycoating, a droplet discharge method (e.g., an inkjet method, screenprinting, or offset printing), roll coating, curtain coating, knifecoating, or the like depending on the material.

With detection of light that enters the photodiode 602, data on anobject to be detected can be read. A light source such as a backlightcan be used for the data reading on the object.

Any of the transistors exemplified in Embodiments 1 to 3 can be used asthe transistor 640. The transistor including the oxide semiconductorfilm which is highly purified by removing impurities such as hydrogen,moisture, a hydroxyl group, or hydride (also referred to as a hydrogencompound) and contains excessive oxygen supplied by oxygen doping or thelike whose variation in the electric characteristics is suppressed iselectrically stable. Accordingly, a highly reliable semiconductor devicecan be provided.

Embodiment 6 can be implemented in appropriate combination with any ofthe structures described in the other embodiments.

Embodiment 7

A semiconductor device disclosed in this specification can be applied toa variety of electronic appliances (including game machines). Examplesof electronic appliances are a television set (also referred to as atelevision or a television receiver), a monitor of a computer or thelike, a camera such as a digital camera or a digital video camera, adigital photo frame, a mobile phone handset (also referred to as amobile phone or a mobile phone device), a portable game machine, aportable information terminal, an audio reproducing device, alarge-sized game machine such as a pachinko machine, and the like.Examples of electronic appliances each including the liquid crystaldisplay device described in the above embodiment will be describedbelow.

FIG. 10A illustrates an electronic book reader (also referred to as ane-book reader) which can include housings 9630, a display portion 9631,operation keys 9632, a solar cell 9633, and a charge and dischargecontrol circuit 9634. The electronic book reader illustrated in FIG. 10Ahas a function of displaying various kinds of information (e.g., a stillimage, a moving image, and a text image) on the display portion, afunction of displaying a calendar, a date, the time, or the like on thedisplay portion, a function of operating or editing the informationdisplayed on the display portion, a function of controlling processingby various kinds of software (programs), and the like. In FIG. 10A, thecharge and discharge control circuit 9634 has a battery 9635 and a DCDCconverter (hereinafter, abbreviated as a converter) 9636. Any of thesemiconductor devices described in the above embodiments can be appliedto the display portion 9631, whereby a highly reliable electronic bookreader can be provided.

In the case where a transflective liquid crystal display device or areflective liquid crystal display device is used as the display portion9631, use under a relatively bright condition is assumed; therefore, thestructure illustrated in FIG. 10A is preferable because power generationby the solar cell 9633 and charge with the battery 9635 are effectivelyperformed. Since the solar cell 9633 can be provided in a space (asurface or a rear surface) of the housing 9630 as appropriate, thebattery 9635 can be efficiently charged, which is preferable. A lithiumion battery may be used as the battery 9635, which provides an advantageof downsizing or the like.

The structure and the operation of the charge and discharge controlcircuit 9634 illustrated in FIG. 10A are described with reference to ablock diagram of FIG. 10B. The solar cell 9633, the battery 9635, theconverter 9636, a converter 9637, switches SW1 to SW3, and the displayportion 9631 are shown in FIG. 10B, and the battery 9635, the converter9636, the converter 9637, and the switches SW1 to SW3 are included inthe charge and discharge control circuit 9634.

First, an example of operation in the case where power is generated bythe solar cell 9633 using external light is described. The voltage ofpower generated by the solar cell is raised or lowered by the converter9636 to a voltage for charging the battery 9635. Then, when the powerfrom the solar cell 9633 is used for the operation of the displayportion 9631, the switch SW1 is turned on and the voltage of the poweris raised or lowered by the converter 9637 to a voltage needed for thedisplay portion 9631. In addition, when display on the display portion9631 is not performed, for example, the switch SW1 is turned off and theswitch SW2 is turned on so that charge of the battery 9635 is performed.

Next, operation in the case where power is not generated by the solarcell 9633 using external light is described. The voltage of poweraccumulated in the battery 9635 is raised or lowered by the converter9637 with the switch SW3 turned on. Then, power from the battery 9635 isused for the operation of the display portion 9631.

Although the solar cell 9633 is described as an example of a means forcharging, the battery 9635 may be charged with another means. The solarcell 9633 may be combined with another means for charging.

FIG. 11A illustrates a laptop personal computer, which includes a mainbody 3001, a housing 3002, a display portion 3003, a keyboard 3004, andthe like. By applying any of the semiconductor devices described in theabove embodiments to the display portion 3003, a highly reliable laptoppersonal computer can be provided.

FIG. 11B is a personal digital assistant (PDA), which includes a mainbody 3021 provided with a display portion 3023, an external interface3025, operation buttons 3024, and the like. A stylus 3022 is included asan accessory for operation. By applying any of the semiconductor devicesdescribed in the above embodiments to the display portion 3023, a highlyreliable personal digital assistant (PDA) can be provided.

FIG. 11C illustrates an example of an electronic book reader. Forexample, an electronic book reader 2700 includes two housings, i.e., ahousing 2701 and a housing 2703. The housing 2701 and the housing 2703are combined with a hinge 2711 so that the electronic book reader 2700can be opened and closed with the hinge 2711 as an axis. With such astructure, the electronic book reader 2700 can operate like a paperbook.

A display portion 2705 and a display portion 2707 are incorporated inthe housing 2701 and the housing 2703, respectively. The display portion2705 and the display portion 2707 may display one image or differentimages. In the structure where different images are displayed ondifferent display portions, for example, the right display portion (thedisplay portion 2705 in FIG. 11C) displays text and the left displayportion (the display portion 2707 in FIG. 11C) displays images. Byapplying any of the semiconductor devices described in the aboveembodiments to the display portion 2705, 2707, a highly reliableelectronic book reader can be provided as the electronic book reader2700.

FIG. 11C illustrates an example in which the housing 2701 is providedwith an operation portion and the like. For example, the housing 2701 isprovided with a power switch 2721, operation keys 2723, a speaker 2725,and the like. With the operation keys 2723, pages can be turned. Akeyboard, a pointing device, or the like may also be provided on thesurface of the housing, on which the display portion is provided.Furthermore, an external connection terminal (an earphone terminal, aUSB terminal, or the like), a recording medium insertion portion, andthe like may be provided on the back surface or the side surface of thehousing. Moreover, the electronic book reader 2700 may be equipped witha function of an electronic dictionary.

The electronic book reader 2700 may have a structure capable ofwirelessly transmitting and receiving data. Through wirelesscommunication, book data or the like can be purchased and downloadedfrom an electronic book server.

FIG. 11D illustrates a mobile phone, which includes two housings, i.e.,a housing 2800 and a housing 2801. The housing 2801 includes a displaypanel 2802, a speaker 2803, a microphone 2804, a pointing device 2806, acamera lens 2807, an external connection terminal 2808, and the like. Inaddition, the housing 2800 includes a solar cell 2810 having a functionof charging the mobile phone, an external memory slot 2811, and thelike. An antenna is incorporated in the housing 2801. By applying any ofthe semiconductor devices described in the above embodiments to thedisplay panel 2802, a highly reliable mobile phone can be provided.

Further, the display panel 2802 is provided with a touch panel. Aplurality of operation keys 2805 which is displayed as images isillustrated by dashed lines in FIG. 11D. A boosting circuit by which avoltage output from the solar cell 2810 is increased to be sufficientlyhigh for each circuit is also provided.

On the display panel 2802, the display direction can be appropriatelychanged depending on a usage pattern. Further, the mobile phone isprovided with the camera lens 2807 on the same surface as the displaypanel 2802, and thus it can be used as a video phone. The speaker 2803and the microphone 2804 can be used for videophone calls, recording andplaying sound, and the like as well as voice calls. Furthermore, thehousings 2800 and 2801 which are developed as illustrated in FIG. 11Dcan overlap with each other by sliding; thus, the size of the mobilephone can be decreased, which makes the mobile phone suitable for beingcarried.

The external connection terminal 2808 can be connected to an AC adapterand various types of cables such as a USB cable, and charging and datacommunication with a personal computer are possible. Moreover, a largeamount of data can be stored and moved with a storage medium insertedinto the external memory slot 2811.

Further, in addition to the above functions, an infrared communicationfunction, a television reception function, or the like may be equipped.

FIG. 11E illustrates a digital video camera which includes a main body3051, a display portion A 3057, an eyepiece portion 3053, an operationswitch 3054, a display portion B 3055, a battery 3056, and the like. Byapplying any of the semiconductor devices described in the aboveembodiments to the display portion A 3057 and/or the display portion B3055, a highly reliable digital video camera can be provided.

FIG. 11F illustrates an example of a television device. In a televisionset 9600, a display portion 9603 is incorporated in a housing 9601. Thedisplay portion 9603 can display images. In FIG. 11F, the housing 9601is supported by a stand 9605. By applying any of the semiconductordevices described in the above embodiments to the display portion 9603,a high reliable television set can be provided as the television set9600.

The television set 9600 can be operated by an operation switch of thehousing 9601 or a separate remote controller. Further, the remotecontroller may be provided with a display portion for displaying dataoutput from the remote controller.

The television set 9600 is provided with a receiver, a modem, and thelike. With the receiver, general television broadcasting can bereceived. Moreover, when the display device is connected to acommunication network with or without wires via the modem, one-way (fromsender to receiver) or two-way (between sender and receiver or betweenreceivers) data communication can be performed.

Embodiment 7 can be implemented in appropriate combination with any ofthe structures described in the other embodiments.

This application is based on Japanese Patent Application serial No.2010-100316 filed with Japan Patent Office on Apr. 23, 2010, the entirecontents of which are hereby incorporated by reference.

1. (canceled)
 2. A semiconductor device comprising: a gate electrode; agate insulating film formed over the gate electrode; an oxidesemiconductor film formed over the gate insulating film, wherein theoxide semiconductor film comprising a channel formation region; a sourceelectrode layer and a drain electrode layer formed over and inelectrical contact with the oxide semiconductor film; a first metaloxide region formed on at least a side surface of the source electrodelayer; a second metal oxide region formed on at least a side surface ofthe drain electrode layer; and a second insulating film formed over thesource electrode layer and the drain electrode layer, wherein the secondinsulating film contacts the first metal oxide region, the second metaloxide region and at least a portion of the oxide semiconductor filmbetween the source electrode layer and the drain electrode layer.
 3. Thesemiconductor device according to claim 2, wherein each of the sourceelectrode layer and the drain electrode layer comprises at least one ofAl, Cr, Cu, Ta, Ti, Mo and W.
 4. The semiconductor device according toclaim 2, wherein each of the source electrode layer and the drainelectrode layer comprises a copper film.
 5. The semiconductor deviceaccording to claim 2, wherein the first metal oxide region is formed onan upper surface of the source electrode layer.
 6. The semiconductordevice according to claim 2, wherein the second metal oxide region isformed on an upper surface of the drain electrode layer.
 7. Asemiconductor device comprising: a gate electrode; a gate insulatingfilm formed over the gate electrode; an oxide semiconductor film formedover the gate insulating film, wherein the oxide semiconductor filmcomprising a channel formation region; a source electrode layer and adrain electrode layer formed over and in electrical contact with theoxide semiconductor film; a first metal oxide region formed on thesource electrode layer; a second metal oxide region formed on the drainelectrode layer; and a second insulating film formed over the sourceelectrode layer and the drain electrode layer, wherein the secondinsulating film contacts the first metal oxide region, the second metaloxide region and at least a portion of the oxide semiconductor filmbetween the source electrode layer and the drain electrode layer.
 8. Thesemiconductor device according to claim 7, wherein each of the sourceelectrode layer and the drain electrode layer comprises at least one ofAl, Cr, Cu, Ta, Ti, Mo and W.
 9. The semiconductor device according toclaim 7, wherein each of the source electrode layer and the drainelectrode layer comprises a copper film.